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+<div class="section-level-extent" id="C_002b_002b-Dialect-Options"> <div class="nav-panel"> <p> Next: <a href="objective-c-and-objective-c_002b_002b-dialect-options" accesskey="n" rel="next">Options Controlling Objective-C and Objective-C++ Dialects</a>, Previous: <a href="c-dialect-options" accesskey="p" rel="prev">Options Controlling C Dialect</a>, Up: <a href="invoking-gcc" accesskey="u" rel="up">GCC Command Options</a> [<a href="index#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="indices" title="Index" rel="index">Index</a>]</p> </div>  <h1 class="section" id="Options-Controlling-C_002b_002b-Dialect"><span>3.5 Options Controlling C++ Dialect<a class="copiable-link" href="#Options-Controlling-C_002b_002b-Dialect"> ¶</a></span></h1>    <p>This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file <samp class="file">firstClass.C</samp> like this: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">g++ -g -fstrict-enums -O -c firstClass.C</pre>
+</div> <p>In this example, only <samp class="option">-fstrict-enums</samp> is an option meant only for C++ programs; you can use the other options with any language supported by GCC. </p> <p>Some options for compiling C programs, such as <samp class="option">-std</samp>, are also relevant for C++ programs. See <a class="xref" href="c-dialect-options">Options Controlling C Dialect</a>. </p> <p>Here is a list of options that are <em class="emph">only</em> for compiling C++ programs: </p> <dl class="table"> <dt>
+<span><code class="code">-fabi-version=<var class="var">n</var></code><a class="copiable-link" href="#index-fabi-version"> ¶</a></span>
+</dt> <dd>
+<p>Use version <var class="var">n</var> of the C++ ABI. The default is version 0. </p> <p>Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. </p> <p>Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. </p> <p>Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9. </p> <p>Version 3 corrects an error in mangling a constant address as a template argument. </p> <p>Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types. </p> <p>Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter. </p> <p>Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and –, and a class scope function used as a template argument. </p> <p>Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope. </p> <p>Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers. </p> <p>Version 9, which first appeared in G++ 5.2, corrects the alignment of <code class="code">nullptr_t</code>. </p> <p>Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. ‘<samp class="samp">stdcall</samp>’). </p> <p>Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies <samp class="option">-fnew-inheriting-ctors</samp>. </p> <p>Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. </p> <p>Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. </p> <p>Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression. </p> <p>Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression. </p> <p>Version 16, which first appeared in G++ 11, changes the mangling of <code class="code">__alignof__</code> to be distinct from that of <code class="code">alignof</code>, and dependent operator names. </p> <p>Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up. </p> <p>Version 18, which first appeard in G++ 13, fixes manglings of lambdas that have additional context. </p> <p>See also <samp class="option">-Wabi</samp>. </p> </dd> <dt>
+<span><code class="code">-fabi-compat-version=<var class="var">n</var></code><a class="copiable-link" href="#index-fabi-compat-version"> ¶</a></span>
+</dt> <dd>
+<p>On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias. </p> <p>With <samp class="option">-fabi-version=0</samp> (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use <samp class="option">-fabi-compat-version=2</samp>. </p> <p>If this option is not provided but <samp class="option">-Wabi=<var class="var">n</var></samp> is, that version is used for compatibility aliases. If this option is provided along with <samp class="option">-Wabi</samp> (without the version), the version from this option is used for the warning. </p> </dd> <dt>
+ <span><code class="code">-fno-access-control</code><a class="copiable-link" href="#index-fno-access-control"> ¶</a></span>
+</dt> <dd>
+<p>Turn off all access checking. This switch is mainly useful for working around bugs in the access control code. </p> </dd> <dt>
+<span><code class="code">-faligned-new</code><a class="copiable-link" href="#index-faligned-new"> ¶</a></span>
+</dt> <dd>
+<p>Enable support for C++17 <code class="code">new</code> of types that require more alignment than <code class="code">void* ::operator new(std::size_t)</code> provides. A numeric argument such as <code class="code">-faligned-new=32</code> can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of <code class="code">alignof(std::max_align_t)</code>. </p> <p>This flag is enabled by default for <samp class="option">-std=c++17</samp>. </p> </dd> <dt>
+ <span><code class="code">-fchar8_t</code><a class="copiable-link" href="#index-fchar8_005ft"> ¶</a></span>
+</dt> <dt><code class="code">-fno-char8_t</code></dt> <dd>
+<p>Enable support for <code class="code">char8_t</code> as adopted for C++20. This includes the addition of a new <code class="code">char8_t</code> fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new <code class="code">__cpp_char8_t</code> and <code class="code">__cpp_lib_char8_t</code> feature test macros. </p> <p>This option enables functions to be overloaded for ordinary and UTF-8 strings: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">int f(const char *);    // #1
+int f(const char8_t *); // #2
+int v1 = f("text");     // Calls #1
+int v2 = f(u8"text");   // Calls #2</pre>
+</div> <p>and introduces new signatures for user-defined literals: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">int operator""_udl1(char8_t);
+int v3 = u8'x'_udl1;
+int operator""_udl2(const char8_t*, std::size_t);
+int v4 = u8"text"_udl2;
+template&lt;typename T, T...&gt; int operator""_udl3();
+int v5 = u8"text"_udl3;</pre>
+</div> <p>The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when <samp class="option">-fchar8_t</samp> is specified. </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">const char *cp = u8"xx";// error: invalid conversion from
+                        //        `const char8_t*' to `const char*'
+int f(const char*);
+auto v = f(u8"xx");     // error: invalid conversion from
+                        //        `const char8_t*' to `const char*'
+std::string s{u8"xx"};  // error: no matching function for call to
+                        //        `std::basic_string&lt;char&gt;::basic_string()'
+using namespace std::literals;
+s = u8"xx"s;            // error: conversion from
+                        //        `basic_string&lt;char8_t&gt;' to non-scalar
+                        //        type `basic_string&lt;char&gt;' requested</pre>
+</div> </dd> <dt>
+<span><code class="code">-fcheck-new</code><a class="copiable-link" href="#index-fcheck-new"> ¶</a></span>
+</dt> <dd>
+<p>Check that the pointer returned by <code class="code">operator new</code> is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that <code class="code">operator new</code> only returns <code class="code">0</code> if it is declared <code class="code">throw()</code>, in which case the compiler always checks the return value even without this option. In all other cases, when <code class="code">operator new</code> has a non-empty exception specification, memory exhaustion is signalled by throwing <code class="code">std::bad_alloc</code>. See also ‘<samp class="samp">new (nothrow)</samp>’. </p> </dd> <dt>
+ <span><code class="code">-fconcepts</code><a class="copiable-link" href="#index-fconcepts"> ¶</a></span>
+</dt> <dt><code class="code">-fconcepts-ts</code></dt> <dd>
+<p>Enable support for the C++ Concepts feature for constraining template arguments. With <samp class="option">-std=c++20</samp> and above, Concepts are part of the language standard, so <samp class="option">-fconcepts</samp> defaults to on. </p> <p>Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn’t make it into the standard, can additionally be enabled by <samp class="option">-fconcepts-ts</samp>. </p> </dd> <dt>
+<span><code class="code">-fconstexpr-depth=<var class="var">n</var></code><a class="copiable-link" href="#index-fconstexpr-depth"> ¶</a></span>
+</dt> <dd>
+<p>Set the maximum nested evaluation depth for C++11 constexpr functions to <var class="var">n</var>. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. </p> </dd> <dt>
+<span><code class="code">-fconstexpr-cache-depth=<var class="var">n</var></code><a class="copiable-link" href="#index-fconstexpr-cache-depth"> ¶</a></span>
+</dt> <dd>
+<p>Set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to <var class="var">n</var>. This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. </p> </dd> <dt>
+<span><code class="code">-fconstexpr-fp-except</code><a class="copiable-link" href="#index-fconstexpr-fp-except"> ¶</a></span>
+</dt> <dd>
+<p>Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined. </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except</pre>
+</div> </dd> <dt>
+<span><code class="code">-fconstexpr-loop-limit=<var class="var">n</var></code><a class="copiable-link" href="#index-fconstexpr-loop-limit"> ¶</a></span>
+</dt> <dd>
+<p>Set the maximum number of iterations for a loop in C++14 constexpr functions to <var class="var">n</var>. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1&lt;&lt;18). </p> </dd> <dt>
+<span><code class="code">-fconstexpr-ops-limit=<var class="var">n</var></code><a class="copiable-link" href="#index-fconstexpr-ops-limit"> ¶</a></span>
+</dt> <dd>
+<p>Set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1&lt;&lt;25). </p> </dd> <dt>
+<span><code class="code">-fcontracts</code><a class="copiable-link" href="#index-fcontracts"> ¶</a></span>
+</dt> <dd>
+<p>Enable experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This functionality is intended mostly for those interested in experimentation towards refining the feature to get it into shape for a future C++ standard. </p> <p>On violation of a checked contract, the violation handler is called. Users can replace the violation handler by defining </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void
+handle_contract_violation (const std::experimental::contract_violation&amp;);</pre>
+</div> <p>There are different sets of additional flags that can be used together to specify which contracts will be checked and how, for N4820 contracts, P1332 contracts, or P1429 contracts; these sets cannot be used together. </p> <dl class="table"> <dt>
+<span><code class="code">-fcontract-mode=[on|off]</code><a class="copiable-link" href="#index-fcontract-mode"> ¶</a></span>
+</dt> <dd>
+<p>Control whether any contracts have any semantics at all. Defaults to on. </p> </dd> <dt>
+<span><code class="code">-fcontract-assumption-mode=[on|off]</code><a class="copiable-link" href="#index-fcontract-assumption-mode"> ¶</a></span>
+</dt> <dd>
+<p>[N4820] Control whether contracts with level ‘<samp class="samp">axiom</samp>’ should have the assume semantic. Defaults to on. </p> </dd> <dt>
+<span><code class="code">-fcontract-build-level=[off|default|audit]</code><a class="copiable-link" href="#index-fcontract-build-level"> ¶</a></span>
+</dt> <dd>
+<p>[N4820] Specify which level of contracts to generate checks for. Defaults to ‘<samp class="samp">default</samp>’. </p> </dd> <dt>
+<span><code class="code">-fcontract-continuation-mode=[on|off]</code><a class="copiable-link" href="#index-fcontract-continuation-mode"> ¶</a></span>
+</dt> <dd>
+<p>[N4820] Control whether to allow the program to continue executing after a contract violation. That is, do checked contracts have the ‘<samp class="samp">maybe</samp>’ semantic described below rather than the ‘<samp class="samp">never</samp>’ semantic. Defaults to off. </p> </dd> <dt>
+<span><code class="code">-fcontract-role=&lt;name&gt;:&lt;default&gt;,&lt;audit&gt;,&lt;axiom&gt;</code><a class="copiable-link" href="#index-fcontract-role"> ¶</a></span>
+</dt> <dd>
+<p>[P1332] Specify the concrete semantics for each contract level of a particular contract role. </p> </dd> <dt><code class="code">-fcontract-semantic=[default|audit|axiom]:&lt;semantic&gt;</code></dt> <dd>
+<p>[P1429] Specify the concrete semantic for a particular contract level. </p> </dd> <dt>
+<span><code class="code">-fcontract-strict-declarations=[on|off]</code><a class="copiable-link" href="#index-fcontract-strict-declarations"> ¶</a></span>
+</dt> <dd><p>Control whether to reject adding contracts to a function after its first declaration. Defaults to off. </p></dd> </dl> <p>The possible concrete semantics for that can be specified with ‘<samp class="samp">-fcontract-role</samp>’ or ‘<samp class="samp">-fcontract-semantic</samp>’ are: </p> <dl class="table"> <dt><code class="code">ignore</code></dt> <dd>
+<p>This contract has no effect. </p> </dd> <dt><code class="code">assume</code></dt> <dd>
+<p>This contract is treated like C++23 <code class="code">[[assume]]</code>. </p> </dd> <dt><code class="code">check_never_continue</code></dt> <dt><code class="code">never</code></dt> <dt><code class="code">abort</code></dt> <dd>
+<p>This contract is checked. If it fails, the violation handler is called. If the handler returns, <code class="code">std::terminate</code> is called. </p> </dd> <dt><code class="code">check_maybe_continue</code></dt> <dt><code class="code">maybe</code></dt> <dd><p>This contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally. </p></dd> </dl> </dd> <dt>
+<span><code class="code">-fcoroutines</code><a class="copiable-link" href="#index-fcoroutines"> ¶</a></span>
+</dt> <dd>
+<p>Enable support for the C++ coroutines extension (experimental). </p> </dd> <dt>
+ <span><code class="code">-fno-elide-constructors</code><a class="copiable-link" href="#index-fno-elide-constructors"> ¶</a></span>
+</dt> <dd>
+<p>The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline. </p> <p>In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. </p> </dd> <dt>
+ <span><code class="code">-fno-enforce-eh-specs</code><a class="copiable-link" href="#index-fno-enforce-eh-specs"> ¶</a></span>
+</dt> <dd>
+<p>Don’t generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining <code class="code">NDEBUG</code>. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. </p> </dd> <dt>
+ <span><code class="code">-fextern-tls-init</code><a class="copiable-link" href="#index-fextern-tls-init"> ¶</a></span>
+</dt> <dt><code class="code">-fno-extern-tls-init</code></dt> <dd>
+<p>The C++11 and OpenMP standards allow <code class="code">thread_local</code> and <code class="code">threadprivate</code> variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn’t actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the <samp class="option">-fno-extern-tls-init</samp> option. </p> <p>On targets that support symbol aliases, the default is <samp class="option">-fextern-tls-init</samp>. On targets that do not support symbol aliases, the default is <samp class="option">-fno-extern-tls-init</samp>. </p> </dd> <dt>
+ <span><code class="code">-ffold-simple-inlines</code><a class="copiable-link" href="#index-ffold-simple-inlines"> ¶</a></span>
+</dt> <dt><code class="code">-fno-fold-simple-inlines</code></dt> <dd>
+<p>Permit the C++ frontend to fold calls to <code class="code">std::move</code>, <code class="code">std::forward</code>, <code class="code">std::addressof</code> and <code class="code">std::as_const</code>. In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless <samp class="option">-fno-inline</samp> is active. </p> </dd> <dt>
+ <span><code class="code">-fno-gnu-keywords</code><a class="copiable-link" href="#index-fno-gnu-keywords"> ¶</a></span>
+</dt> <dd>
+<p>Do not recognize <code class="code">typeof</code> as a keyword, so that code can use this word as an identifier. You can use the keyword <code class="code">__typeof__</code> instead. This option is implied by the strict ISO C++ dialects: <samp class="option">-ansi</samp>, <samp class="option">-std=c++98</samp>, <samp class="option">-std=c++11</samp>, etc. </p> </dd> <dt>
+<span><code class="code">-fimplicit-constexpr</code><a class="copiable-link" href="#index-fimplicit-constexpr"> ¶</a></span>
+</dt> <dd>
+<p>Make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations. </p> </dd> <dt>
+ <span><code class="code">-fno-implicit-templates</code><a class="copiable-link" href="#index-fno-implicit-templates"> ¶</a></span>
+</dt> <dd>
+<p>Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. See <a class="xref" href="template-instantiation">Where’s the Template?</a>, for more information. </p> </dd> <dt>
+ <span><code class="code">-fno-implicit-inline-templates</code><a class="copiable-link" href="#index-fno-implicit-inline-templates"> ¶</a></span>
+</dt> <dd>
+<p>Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations. </p> </dd> <dt>
+ <span><code class="code">-fno-implement-inlines</code><a class="copiable-link" href="#index-fno-implement-inlines"> ¶</a></span>
+</dt> <dd>
+<p>To save space, do not emit out-of-line copies of inline functions controlled by <code class="code">#pragma implementation</code>. This causes linker errors if these functions are not inlined everywhere they are called. </p> </dd> <dt>
+ <span><code class="code">-fmodules-ts</code><a class="copiable-link" href="#index-fmodules-ts"> ¶</a></span>
+</dt> <dt><code class="code">-fno-modules-ts</code></dt> <dd>
+<p>Enable support for C++20 modules (see <a class="pxref" href="c_002b_002b-modules">C++ Modules</a>). The <samp class="option">-fno-modules-ts</samp> is usually not needed, as that is the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. </p> </dd> <dt>
+<span><code class="code">-fmodule-header</code><a class="copiable-link" href="#index-fmodule-header"> ¶</a></span>
+</dt> <dt><code class="code">-fmodule-header=user</code></dt> <dt><code class="code">-fmodule-header=system</code></dt> <dd>
+<p>Compile a header file to create an importable header unit. </p> </dd> <dt>
+<span><code class="code">-fmodule-implicit-inline</code><a class="copiable-link" href="#index-fmodule-implicit-inline"> ¶</a></span>
+</dt> <dd>
+<p>Member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.) </p> </dd> <dt>
+ <span><code class="code">-fno-module-lazy</code><a class="copiable-link" href="#index-fno-module-lazy"> ¶</a></span>
+</dt> <dd>
+<p>Disable lazy module importing and module mapper creation. </p> </dd> <dt>
+ <span><code class="code">-fmodule-mapper=<span class="r">[</span><var class="var">hostname</var><span class="r">]</span>:<var class="var">port</var><span class="r">[</span>?<var class="var">ident</var><span class="r">]</span></code><a class="copiable-link" href="#index-CXX_005fMODULE_005fMAPPER-environment-variable"> ¶</a></span>
+</dt> <dt><code class="code">-fmodule-mapper=|<var class="var">program</var><span class="r">[</span>?<var class="var">ident</var><span class="r">]</span> <var class="var">args...</var></code></dt> <dt><code class="code">-fmodule-mapper==<var class="var">socket</var><span class="r">[</span>?<var class="var">ident</var><span class="r">]</span></code></dt> <dt><code class="code">-fmodule-mapper=&lt;&gt;<span class="r">[</span><var class="var">inout</var><span class="r">]</span><span class="r">[</span>?<var class="var">ident</var><span class="r">]</span></code></dt> <dt><code class="code">-fmodule-mapper=&lt;<var class="var">in</var>&gt;<var class="var">out</var><span class="r">[</span>?<var class="var">ident</var><span class="r">]</span></code></dt> <dt><code class="code">-fmodule-mapper=<var class="var">file</var><span class="r">[</span>?<var class="var">ident</var><span class="r">]</span></code></dt> <dd>
+<p>An oracle to query for module name to filename mappings. If unspecified the <code class="env">CXX_MODULE_MAPPER</code> environment variable is used, and if that is unset, an in-process default is provided. </p> </dd> <dt>
+<span><code class="code">-fmodule-only</code><a class="copiable-link" href="#index-fmodule-only"> ¶</a></span>
+</dt> <dd>
+<p>Only emit the Compiled Module Interface, inhibiting any object file. </p> </dd> <dt>
+<span><code class="code">-fms-extensions</code><a class="copiable-link" href="#index-fms-extensions-1"> ¶</a></span>
+</dt> <dd>
+<p>Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax. </p> </dd> <dt>
+<span><code class="code">-fnew-inheriting-ctors</code><a class="copiable-link" href="#index-fnew-inheriting-ctors"> ¶</a></span>
+</dt> <dd>
+<p>Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless <samp class="option">-fabi-version=10</samp> or lower is specified. </p> </dd> <dt>
+<span><code class="code">-fnew-ttp-matching</code><a class="copiable-link" href="#index-fnew-ttp-matching"> ¶</a></span>
+</dt> <dd>
+<p>Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for <samp class="option">-std=c++17</samp>. </p> </dd> <dt>
+ <span><code class="code">-fno-nonansi-builtins</code><a class="copiable-link" href="#index-fno-nonansi-builtins"> ¶</a></span>
+</dt> <dd>
+<p>Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include <code class="code">ffs</code>, <code class="code">alloca</code>, <code class="code">_exit</code>, <code class="code">index</code>, <code class="code">bzero</code>, <code class="code">conjf</code>, and other related functions. </p> </dd> <dt>
+<span><code class="code">-fnothrow-opt</code><a class="copiable-link" href="#index-fnothrow-opt"> ¶</a></span>
+</dt> <dd>
+<p>Treat a <code class="code">throw()</code> exception specification as if it were a <code class="code">noexcept</code> specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to <code class="code">terminate</code> rather than <code class="code">unexpected</code>. </p> </dd> <dt>
+ <span><code class="code">-fno-operator-names</code><a class="copiable-link" href="#index-fno-operator-names"> ¶</a></span>
+</dt> <dd>
+<p>Do not treat the operator name keywords <code class="code">and</code>, <code class="code">bitand</code>, <code class="code">bitor</code>, <code class="code">compl</code>, <code class="code">not</code>, <code class="code">or</code> and <code class="code">xor</code> as synonyms as keywords. </p> </dd> <dt>
+ <span><code class="code">-fno-optional-diags</code><a class="copiable-link" href="#index-fno-optional-diags"> ¶</a></span>
+</dt> <dd>
+<p>Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class. </p> </dd> <dt>
+<span><code class="code">-fpermissive</code><a class="copiable-link" href="#index-fpermissive"> ¶</a></span>
+</dt> <dd>
+<p>Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using <samp class="option">-fpermissive</samp> allows some nonconforming code to compile. </p> </dd> <dt>
+ <span><code class="code">-fno-pretty-templates</code><a class="copiable-link" href="#index-fno-pretty-templates"> ¶</a></span>
+</dt> <dd>
+<p>When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. <code class="code">void f(T) [with T = int]</code> rather than <code class="code">void f(int)</code>) so that it’s clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use <samp class="option">-fno-pretty-templates</samp> to disable them. </p> </dd> <dt>
+ <span><code class="code">-fno-rtti</code><a class="copiable-link" href="#index-fno-rtti"> ¶</a></span>
+</dt> <dd>
+<p>Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (<code class="code">dynamic_cast</code> and <code class="code">typeid</code>). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The <code class="code">dynamic_cast</code> operator can still be used for casts that do not require run-time type information, i.e. casts to <code class="code">void *</code> or to unambiguous base classes. </p> <p>Mixing code compiled with <samp class="option">-frtti</samp> with that compiled with <samp class="option">-fno-rtti</samp> may not work. For example, programs may fail to link if a class compiled with <samp class="option">-fno-rtti</samp> is used as a base for a class compiled with <samp class="option">-frtti</samp>. </p> </dd> <dt>
+<span><code class="code">-fsized-deallocation</code><a class="copiable-link" href="#index-fsized-deallocation"> ¶</a></span>
+</dt> <dd>
+<p>Enable the built-in global declarations </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void operator delete (void *, std::size_t) noexcept;
+void operator delete[] (void *, std::size_t) noexcept;</pre>
+</div> <p>as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under <samp class="option">-std=c++14</samp> and above. The flag <samp class="option">-Wsized-deallocation</samp> warns about places that might want to add a definition. </p> </dd> <dt>
+<span><code class="code">-fstrict-enums</code><a class="copiable-link" href="#index-fstrict-enums"> ¶</a></span>
+</dt> <dd>
+<p>Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. </p> </dd> <dt>
+<span><code class="code">-fstrong-eval-order</code><a class="copiable-link" href="#index-fstrong-eval-order"> ¶</a></span>
+</dt> <dd>
+<p>Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with <samp class="option">-std=c++17</samp>. <samp class="option">-fstrong-eval-order=some</samp> enables just the ordering of member access and shift expressions, and is the default without <samp class="option">-std=c++17</samp>. </p> </dd> <dt>
+<span><code class="code">-ftemplate-backtrace-limit=<var class="var">n</var></code><a class="copiable-link" href="#index-ftemplate-backtrace-limit"> ¶</a></span>
+</dt> <dd>
+<p>Set the maximum number of template instantiation notes for a single warning or error to <var class="var">n</var>. The default value is 10. </p> </dd> <dt>
+<span><code class="code">-ftemplate-depth=<var class="var">n</var></code><a class="copiable-link" href="#index-ftemplate-depth"> ¶</a></span>
+</dt> <dd>
+<p>Set the maximum instantiation depth for template classes to <var class="var">n</var>. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. </p> </dd> <dt>
+ <span><code class="code">-fno-threadsafe-statics</code><a class="copiable-link" href="#index-fno-threadsafe-statics"> ¶</a></span>
+</dt> <dd>
+<p>Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn’t need to be thread-safe. </p> </dd> <dt>
+<span><code class="code">-fuse-cxa-atexit</code><a class="copiable-link" href="#index-fuse-cxa-atexit"> ¶</a></span>
+</dt> <dd>
+<p>Register destructors for objects with static storage duration with the <code class="code">__cxa_atexit</code> function rather than the <code class="code">atexit</code> function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports <code class="code">__cxa_atexit</code>. </p> </dd> <dt>
+ <span><code class="code">-fno-use-cxa-get-exception-ptr</code><a class="copiable-link" href="#index-fno-use-cxa-get-exception-ptr"> ¶</a></span>
+</dt> <dd>
+<p>Don’t use the <code class="code">__cxa_get_exception_ptr</code> runtime routine. This causes <code class="code">std::uncaught_exception</code> to be incorrect, but is necessary if the runtime routine is not available. </p> </dd> <dt>
+<span><code class="code">-fvisibility-inlines-hidden</code><a class="copiable-link" href="#index-fvisibility-inlines-hidden"> ¶</a></span>
+</dt> <dd>
+<p>This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. </p> <p>The effect of this is that GCC may, effectively, mark inline methods with <code class="code">__attribute__ ((visibility ("hidden")))</code> so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. </p> <p>The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object. </p> <p>You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect. </p> <p>Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See <a class="xref" href="template-instantiation">Where’s the Template?</a>. </p> </dd> <dt>
+<span><code class="code">-fvisibility-ms-compat</code><a class="copiable-link" href="#index-fvisibility-ms-compat"> ¶</a></span>
+</dt> <dd>
+<p>This flag attempts to use visibility settings to make GCC’s C++ linkage model compatible with that of Microsoft Visual Studio. </p> <p>The flag makes these changes to GCC’s linkage model: </p> <ol class="enumerate"> <li> It sets the default visibility to <code class="code">hidden</code>, like <samp class="option">-fvisibility=hidden</samp>. </li>
+<li> Types, but not their members, are not hidden by default. </li>
+<li> The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used. </li>
+</ol> <p>In new code it is better to use <samp class="option">-fvisibility=hidden</samp> and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior. </p> <p>Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. </p> </dd> <dt>
+ <span><code class="code">-fno-weak</code><a class="copiable-link" href="#index-fno-weak"> ¶</a></span>
+</dt> <dd>
+<p>Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. </p> </dd> <dt>
+ <span><code class="code">-fext-numeric-literals <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-fext-numeric-literals"> ¶</a></span>
+</dt> <dd>
+<p>Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: <samp class="option">-std=c++98</samp>, <samp class="option">-std=gnu++98</samp>, <samp class="option">-std=gnu++11</samp>, <samp class="option">-std=gnu++14</samp>. This option is off by default for ISO C++11 onwards (<samp class="option">-std=c++11</samp>, ...). </p> </dd> <dt>
+<span><code class="code">-nostdinc++</code><a class="copiable-link" href="#index-nostdinc_002b_002b"> ¶</a></span>
+</dt> <dd>
+<p>Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) </p> </dd> <dt>
+ <span><code class="code">-flang-info-include-translate</code><a class="copiable-link" href="#index-flang-info-include-translate"> ¶</a></span>
+</dt> <dt><code class="code">-flang-info-include-translate-not</code></dt> <dt><code class="code">-flang-info-include-translate=<var class="var">header</var></code></dt> <dd>
+<p>Inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The <var class="var">header</var> form will inform of include translations relating to that specific header. If <var class="var">header</var> is of the form <code class="code">"user"</code> or <code class="code">&lt;system&gt;</code> it will be resolved to a specific user or system header using the include path. </p> </dd> <dt>
+<span><code class="code">-flang-info-module-cmi</code><a class="copiable-link" href="#index-flang-info-module-cmi"> ¶</a></span>
+</dt> <dt><code class="code">-flang-info-module-cmi=<var class="var">module</var></code></dt> <dd>
+<p>Inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The <var class="var">module</var> form will not reading a specific module’s CMI. <var class="var">module</var> may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in <code class="code">&lt;&gt;</code> or <code class="code">""</code>). </p> </dd> <dt>
+<span><code class="code">-stdlib=<var class="var">libstdc++,libc++</var></code><a class="copiable-link" href="#index-stdlib"> ¶</a></span>
+</dt> <dd><p>When G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available: <var class="var">libstdc++</var> (the default, native C++ runtime for G++) and <var class="var">libc++</var> which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit <code class="code">-lstdc++</code> or <code class="code">-lc++</code> respectively, when a C++ runtime is required for linking. </p></dd> </dl> <p>In addition, these warning options have meanings only for C++ programs: </p> <dl class="table"> <dt>
+<span><code class="code">-Wabi-tag <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wabi-tag"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See <a class="ref" href="c_002b_002b-attributes">C++-Specific Variable, Function, and Type Attributes</a> for more information about ABI tags. </p> </dd> <dt>
+ <span><code class="code">-Wcomma-subscript <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wcomma-subscript"> ¶</a></span>
+</dt> <dd>
+<p>Warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in <code class="code">( )</code> is not deprecated. Example: </p> <div class="example smallexample"> <div class="group"><pre class="example-preformatted" data-language="cpp">void f(int *a, int b, int c) {
+    a[b,c];     // deprecated in C++20, invalid in C++23
+    a[(b,c)];   // OK
+}</pre></div>
+</div> <p>In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with <samp class="option">-Wno-comma-subscript</samp>. </p> <p>Enabled by default with <samp class="option">-std=c++20</samp> unless <samp class="option">-Wno-deprecated</samp>, and with <samp class="option">-std=c++23</samp> regardless of <samp class="option">-Wno-deprecated</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wctad-maybe-unsupported <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wctad-maybe-unsupported"> ¶</a></span>
+</dt> <dd>
+<p>Warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct allow_ctad_t; // any name works
+template &lt;typename T&gt; struct S {
+  S(T) { }
+};
+// Guide with incomplete parameter type will never be considered.
+S(allow_ctad_t) -&gt; S&lt;void&gt;;</pre>
+</div> </dd> <dt>
+ <span><code class="code">-Wctor-dtor-privacy <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wctor-dtor-privacy"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there’s at least one private member function that isn’t a constructor or destructor. </p> </dd> <dt>
+ <span><code class="code">-Wdangling-reference <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wdangling-reference"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a reference is bound to a temporary whose lifetime has ended. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">int n = 1;
+const int&amp; r = std::max(n - 1, n + 1); // r is dangling</pre>
+</div> <p>In the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference <code class="code">r</code> is dangling. This warning also detects dangling references in member initializer lists: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">const int&amp; f(const int&amp; i) { return i; }
+struct S {
+  const int &amp;r; // r is dangling
+  S() : r(f(10)) { }
+};</pre>
+</div> <p>Member functions are checked as well, but only their object argument: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct S {
+   const S&amp; self () { return *this; }
+};
+const S&amp; s = S().self(); // s is dangling</pre>
+</div> <p>Certain functions are safe in this respect, for example <code class="code">std::use_facet</code>: they take and return a reference, but they don’t return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a <code class="code">#pragma</code>: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdangling-reference"
+const T&amp; foo (const T&amp;) { … }
+#pragma GCC diagnostic pop</pre>
+</div> <p><samp class="option">-Wdangling-reference</samp> also warns about code like </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">auto p = std::minmax(1, 2);</pre>
+</div> <p>where <code class="code">std::minmax</code> returns <code class="code">std::pair&lt;const int&amp;, const int&amp;&gt;</code>, and both references dangle after the end of the full expression that contains the call to <code class="code">std::minmax</code>. </p> <p>This warning is enabled by <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wdelete-non-virtual-dtor <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wdelete-non-virtual-dtor"> ¶</a></span>
+</dt> <dd>
+<p>Warn when <code class="code">delete</code> is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wdeprecated-copy <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wdeprecated-copy"> ¶</a></span>
+</dt> <dd>
+<p>Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by <samp class="option">-Wextra</samp>. With <samp class="option">-Wdeprecated-copy-dtor</samp>, also deprecate if the class has a user-provided destructor. </p> </dd> <dt>
+ <span><code class="code">-Wno-deprecated-enum-enum-conversion <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wdeprecated-enum-enum-conversion"> ¶</a></span>
+</dt> <dd>
+<p>Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">enum E1 { e };
+enum E2 { f };
+int k = f - e;</pre>
+</div> <p><samp class="option">-Wdeprecated-enum-enum-conversion</samp> is enabled by default with <samp class="option">-std=c++20</samp>. In pre-C++20 dialects, this warning can be enabled by <samp class="option">-Wenum-conversion</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wno-deprecated-enum-float-conversion <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wdeprecated-enum-float-conversion"> ¶</a></span>
+</dt> <dd>
+<p>Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">enum E1 { e };
+enum E2 { f };
+bool b = e &lt;= 3.7;</pre>
+</div> <p><samp class="option">-Wdeprecated-enum-float-conversion</samp> is enabled by default with <samp class="option">-std=c++20</samp>. In pre-C++20 dialects, this warning can be enabled by <samp class="option">-Wenum-conversion</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wno-init-list-lifetime <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Winit-list-lifetime"> ¶</a></span>
+</dt> <dd>
+<p>Do not warn about uses of <code class="code">std::initializer_list</code> that are likely to result in dangling pointers. Since the underlying array for an <code class="code">initializer_list</code> is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array’s lifetime. For example: </p> <ul class="itemize mark-bullet"> <li>If a function returns a temporary <code class="code">initializer_list</code>, or a local <code class="code">initializer_list</code> variable, the array’s lifetime ends at the end of the return statement, so the value returned has a dangling pointer. </li>
+<li>If a new-expression creates an <code class="code">initializer_list</code>, the array only lives until the end of the enclosing full-expression, so the <code class="code">initializer_list</code> in the heap has a dangling pointer. </li>
+<li>When an <code class="code">initializer_list</code> variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the <code class="code">initializer_list</code> variable has a dangling pointer. <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">// li's initial underlying array lives as long as li
+std::initializer_list&lt;int&gt; li = { 1,2,3 };
+// assignment changes li to point to a temporary array
+li = { 4, 5 };
+// now the temporary is gone and li has a dangling pointer
+int i = li.begin()[0] // undefined behavior</pre>
+</div> </li>
+<li>When a list constructor stores the <code class="code">begin</code> pointer from the <code class="code">initializer_list</code> argument, this doesn’t extend the lifetime of the array, so if a class variable is constructed from a temporary <code class="code">initializer_list</code>, the pointer is left dangling by the end of the variable declaration statement. </li>
+</ul> </dd> <dt>
+ <span><code class="code">-Winvalid-constexpr</code><a class="copiable-link" href="#index-Winvalid-constexpr"> ¶</a></span>
+</dt> <dd> <p>Warn when a function never produces a constant expression. In C++20 and earlier, for every <code class="code">constexpr</code> function and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it’s possible to have a function or a function template marked <code class="code">constexpr</code> for which no invocation satisfies the requirements of a core constant expression. </p> <p>This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, <samp class="option">-Winvalid-constexpr</samp> can be turned on, in which case it will be an ordinary warning. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void f (int&amp; i);
+constexpr void
+g (int&amp; i)
+{
+  // Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
+  f(i);
+}</pre>
+</div> </dd> <dt>
+ <span><code class="code">-Winvalid-imported-macros</code><a class="copiable-link" href="#index-Winvalid-imported-macros"> ¶</a></span>
+</dt> <dd>
+<p>Verify all imported macro definitions are valid at the end of compilation. This is not enabled by default, as it requires additional processing to determine. It may be useful when preparing sets of header-units to ensure consistent macros. </p> </dd> <dt>
+ <span><code class="code">-Wno-literal-suffix <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wliteral-suffix"> ¶</a></span>
+</dt> <dd>
+<p>Do not warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from <code class="code">&lt;inttypes.h&gt;</code>. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">#define __STDC_FORMAT_MACROS
+#include &lt;inttypes.h&gt;
+#include &lt;stdio.h&gt;
+
+int main() {
+  int64_t i64 = 123;
+  printf("My int64: %" PRId64"\n", i64);
+}</pre>
+</div> <p>In this case, <code class="code">PRId64</code> is treated as a separate preprocessing token. </p> <p>This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn’t begin with an underscore. Literal suffix identifiers that don’t begin with an underscore are reserved for future standardization. </p> <p>These warnings are enabled by default. </p> </dd> <dt>
+ <span><code class="code">-Wno-narrowing <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wnarrowing"> ¶</a></span>
+</dt> <dd>
+<p>For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but <samp class="option">-Wno-narrowing</samp> suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. </p> <p>With <samp class="option">-Wnarrowing</samp> in C++98, warn when a narrowing conversion prohibited by C++11 occurs within ‘<samp class="samp">{ }</samp>’, e.g. </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">int i = { 2.2 }; // error: narrowing from double to int</pre>
+</div> <p>This flag is included in <samp class="option">-Wall</samp> and <samp class="option">-Wc++11-compat</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wnoexcept <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wnoexcept"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. <code class="code">throw()</code> or <code class="code">noexcept</code>) but is known by the compiler to never throw an exception. </p> </dd> <dt>
+ <span><code class="code">-Wnoexcept-type <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wnoexcept-type"> ¶</a></span>
+</dt> <dd>
+<p>Warn if the C++17 feature making <code class="code">noexcept</code> part of a function type changes the mangled name of a symbol relative to C++14. Enabled by <samp class="option">-Wabi</samp> and <samp class="option">-Wc++17-compat</samp>. </p> <p>As an example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">template &lt;class T&gt; void f(T t) { t(); };
+void g() noexcept;
+void h() { f(g); }</pre>
+</div> <p>In C++14, <code class="code">f</code> calls <code class="code">f&lt;void(*)()&gt;</code>, but in C++17 it calls <code class="code">f&lt;void(*)()noexcept&gt;</code>. </p> </dd> <dt>
+ <span><code class="code">-Wclass-memaccess <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wclass-memaccess"> ¶</a></span>
+</dt> <dd>
+<p>Warn when the destination of a call to a raw memory function such as <code class="code">memset</code> or <code class="code">memcpy</code> is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to <code class="code">memset</code> below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available. </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">std::string str = "abc";
+memset (&amp;str, 0, sizeof str);</pre>
+</div> <p>The <samp class="option">-Wclass-memaccess</samp> option is enabled by <samp class="option">-Wall</samp>. Explicitly casting the pointer to the class object to <code class="code">void *</code> or to a type that can be safely accessed by the raw memory function suppresses the warning. </p> </dd> <dt>
+ <span><code class="code">-Wnon-virtual-dtor <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wnon-virtual-dtor"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if <samp class="option">-Weffc++</samp> is specified. The <samp class="option">-Wdelete-non-virtual-dtor</samp> option (enabled by <samp class="option">-Wall</samp>) should be preferred because it warns about the unsafe cases without false positives. </p> </dd> <dt>
+ <span><code class="code">-Wregister <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wregister"> ¶</a></span>
+</dt> <dd>
+<p>Warn on uses of the <code class="code">register</code> storage class specifier, except when it is part of the GNU <a class="ref" href="explicit-register-variables">Variables in Specified Registers</a> extension. The use of the <code class="code">register</code> keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with <samp class="option">-std=c++17</samp>. </p> </dd> <dt>
+   <span><code class="code">-Wreorder <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wreorder"> ¶</a></span>
+</dt> <dd>
+<p>Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct A {
+  int i;
+  int j;
+  A(): j (0), i (1) { }
+};</pre>
+</div> <p>The compiler rearranges the member initializers for <code class="code">i</code> and <code class="code">j</code> to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wno-pessimizing-move <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wpessimizing-move"> ¶</a></span>
+</dt> <dd>
+<p>This warning warns when a call to <code class="code">std::move</code> prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type. </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct T {
+…
+};
+T fn()
+{
+  T t;
+  …
+  return std::move (t);
+}</pre>
+</div> <p>But in this example, the <code class="code">std::move</code> call prevents copy elision. </p> <p>This warning is enabled by <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wno-redundant-move <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wredundant-move"> ¶</a></span>
+</dt> <dd>
+<p>This warning warns about redundant calls to <code class="code">std::move</code>; that is, when a move operation would have been performed even without the <code class="code">std::move</code> call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn’t applicable. Consider: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct T {
+…
+};
+T fn(T t)
+{
+  …
+  return std::move (t);
+}</pre>
+</div> <p>Here, the <code class="code">std::move</code> call is redundant. Because G++ implements Core Issue 1579, another example is: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct T { // convertible to U
+…
+};
+struct U {
+…
+};
+U fn()
+{
+  T t;
+  …
+  return std::move (t);
+}</pre>
+</div> <p>In this example, copy elision isn’t applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue. </p> <p>This warning is enabled by <samp class="option">-Wextra</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wrange-loop-construct <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wrange-loop-construct"> ¶</a></span>
+</dt> <dd>
+<p>This warning warns when a C++ range-based for-loop is creating an unnecessary copy. This can happen when the range declaration is not a reference, but probably should be. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct S { char arr[128]; };
+void fn () {
+  S arr[5];
+  for (const auto x : arr) { … }
+}</pre>
+</div> <p>It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. </p> <p>This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void fn() {
+  int arr[10];
+  for (const double &amp;x : arr) { … }
+}</pre>
+</div> <p>In the example above, in every iteration of the loop a temporary value of type <code class="code">double</code> is created and destroyed, to which the reference <code class="code">const double &amp;</code> is bound. </p> <p>This warning is enabled by <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wredundant-tags <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wredundant-tags"> ¶</a></span>
+</dt> <dd>
+<p>Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct foo;
+struct foo *p;   // warn that keyword struct can be eliminated</pre>
+</div> <p>On the other hand, in this example there is no warning: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct foo;
+void foo ();   // "hides" struct foo
+void bar (struct foo&amp;);  // no warning, keyword struct is necessary</pre>
+</div> </dd> <dt>
+ <span><code class="code">-Wno-subobject-linkage <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wsubobject-linkage"> ¶</a></span>
+</dt> <dd>
+<p>Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn’t give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. <samp class="option">-Wsubobject-linkage</samp> is enabled by default. </p> </dd> <dt>
+ <span><code class="code">-Weffc++ <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Weffc_002b_002b"> ¶</a></span>
+</dt> <dd>
+<p>Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ series of books: </p> <ul class="itemize mark-bullet"> <li>Define a copy constructor and an assignment operator for classes with dynamically-allocated memory. </li>
+<li>Prefer initialization to assignment in constructors. </li>
+<li>Have <code class="code">operator=</code> return a reference to <code class="code">*this</code>. </li>
+<li>Don’t try to return a reference when you must return an object. </li>
+<li>Distinguish between prefix and postfix forms of increment and decrement operators. </li>
+<li>Never overload <code class="code">&amp;&amp;</code>, <code class="code">||</code>, or <code class="code">,</code>. </li>
+</ul> <p>This option also enables <samp class="option">-Wnon-virtual-dtor</samp>, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too. </p> <p>When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use ‘<samp class="samp">grep -v</samp>’ to filter out those warnings. </p> </dd> <dt>
+ <span><code class="code">-Wno-exceptions <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wexceptions"> ¶</a></span>
+</dt> <dd>
+<p>Disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers. </p> </dd> <dt>
+ <span><code class="code">-Wstrict-null-sentinel <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wstrict-null-sentinel"> ¶</a></span>
+</dt> <dd>
+<p>Warn about the use of an uncasted <code class="code">NULL</code> as sentinel. When compiling only with GCC this is a valid sentinel, as <code class="code">NULL</code> is defined to <code class="code">__null</code>. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. </p> </dd> <dt>
+ <span><code class="code">-Wno-non-template-friend <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wno-non-template-friend"> ¶</a></span>
+</dt> <dd>
+<p>Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as ‘<samp class="samp">friend int foo(int)</samp>’, where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default. </p> </dd> <dt>
+ <span><code class="code">-Wold-style-cast <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wold-style-cast"> ¶</a></span>
+</dt> <dd>
+<p>Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (<code class="code">dynamic_cast</code>, <code class="code">static_cast</code>, <code class="code">reinterpret_cast</code>, and <code class="code">const_cast</code>) are less vulnerable to unintended effects and much easier to search for. </p> </dd> <dt>
+   <span><code class="code">-Woverloaded-virtual <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Woverloaded-virtual"> ¶</a></span>
+</dt> <dt><code class="code">-Woverloaded-virtual=<var class="var">n</var></code></dt> <dd>
+<p>Warn when a function declaration hides virtual functions from a base class. For example, in: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct A {
+  virtual void f();
+};
+
+struct B: public A {
+  void f(int); // does not override
+};</pre>
+</div> <p>the <code class="code">A</code> class version of <code class="code">f</code> is hidden in <code class="code">B</code>, and code like: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">B* b;
+b-&gt;f();</pre>
+</div> <p>fails to compile. </p> <p>In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add <code class="code">using A::f;</code> to <code class="code">B</code>. </p> <p>The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct C {
+  virtual void f();
+  virtual void f(int);
+};
+
+struct D: public C {
+  void f(int); // does override
+}</pre>
+</div> <p>This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine. </p> <p>At level 1, this case does not warn; at level 2, it does. <samp class="option">-Woverloaded-virtual</samp> by itself selects level 2. Level 1 is included in <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wno-pmf-conversions <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wno-pmf-conversions"> ¶</a></span>
+</dt> <dd>
+<p>Disable the diagnostic for converting a bound pointer to member function to a plain pointer. </p> </dd> <dt>
+ <span><code class="code">-Wsign-promo <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wsign-promo"> ¶</a></span>
+</dt> <dd>
+<p>Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. </p> </dd> <dt>
+ <span><code class="code">-Wtemplates <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wtemplates"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates. </p> </dd> <dt>
+ <span><code class="code">-Wmismatched-new-delete <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wmismatched-new-delete"> ¶</a></span>
+</dt> <dd>
+<p>Warn for mismatches between calls to <code class="code">operator new</code> or <code class="code">operator
+delete</code> and the corresponding call to the allocation or deallocation function. This includes invocations of C++ <code class="code">operator delete</code> with pointers returned from either mismatched forms of <code class="code">operator new</code>, or from other functions that allocate objects for which the <code class="code">operator delete</code> isn’t a suitable deallocator, as well as calls to other deallocation functions with pointers returned from <code class="code">operator new</code> for which the deallocation function isn’t suitable. </p> <p>For example, the <code class="code">delete</code> expression in the function below is diagnosed because it doesn’t match the array form of the <code class="code">new</code> expression the pointer argument was returned from. Similarly, the call to <code class="code">free</code> is also diagnosed. </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void f ()
+{
+  int *a = new int[n];
+  delete a;   // warning: mismatch in array forms of expressions
+
+  char *p = new char[n];
+  free (p);   // warning: mismatch between new and free
+}</pre>
+</div> <p>The related option <samp class="option">-Wmismatched-dealloc</samp> diagnoses mismatches involving allocation and deallocation functions other than <code class="code">operator
+new</code> and <code class="code">operator delete</code>. </p> <p><samp class="option">-Wmismatched-new-delete</samp> is included in <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wmismatched-tags <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wmismatched-tags"> ¶</a></span>
+</dt> <dd>
+<p>Warn for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided. </p> <p>For example, the declaration of <code class="code">struct Object</code> in the argument list of <code class="code">draw</code> triggers the warning. To avoid it, either remove the redundant class-key <code class="code">struct</code> or replace it with <code class="code">class</code> to match its definition. </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">class Object {
+public:
+  virtual ~Object () = 0;
+};
+void draw (struct Object*);</pre>
+</div> <p>It is not wrong to declare a class with the class-key <code class="code">struct</code> as the example above shows. The <samp class="option">-Wmismatched-tags</samp> option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with <samp class="option">-Wredundant-tags</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wmultiple-inheritance <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wmultiple-inheritance"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance. </p> </dd> <dt>
+ <span><code class="code">-Wvirtual-inheritance</code><a class="copiable-link" href="#index-Wvirtual-inheritance"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance. </p> </dd> <dt>
+ <span><code class="code">-Wno-virtual-move-assign</code><a class="copiable-link" href="#index-Wvirtual-move-assign"> ¶</a></span>
+</dt> <dd>
+<p>Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. </p> </dd> <dt>
+ <span><code class="code">-Wnamespaces</code><a class="copiable-link" href="#index-Wnamespaces"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names. </p> </dd> <dt>
+ <span><code class="code">-Wno-terminate <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wterminate"> ¶</a></span>
+</dt> <dd>
+<p>Disable the warning about a throw-expression that will immediately result in a call to <code class="code">terminate</code>. </p> </dd> <dt>
+ <span><code class="code">-Wno-vexing-parse <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wvexing-parse"> ¶</a></span>
+</dt> <dd>
+<p>Warn about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void f(double a) {
+  int i();        // extern int i (void);
+  int n(int(a));  // extern int n (int);
+}</pre>
+</div> <p>Another example: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct S { S(int); };
+void f(double a) {
+  S x(int(a));   // extern struct S x (int);
+  S y(int());    // extern struct S y (int (*) (void));
+  S z();         // extern struct S z (void);
+}</pre>
+</div> <p>The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead. </p> <p>This warning is enabled by default. </p> </dd> <dt>
+ <span><code class="code">-Wno-class-conversion <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wno-class-conversion"> ¶</a></span>
+</dt> <dd>
+<p>Do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called. </p> </dd> <dt>
+ <span><code class="code">-Wvolatile <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wvolatile"> ¶</a></span>
+</dt> <dd>
+<p>Warn about deprecated uses of the <code class="code">volatile</code> qualifier. This includes postfix and prefix <code class="code">++</code> and <code class="code">--</code> expressions of <code class="code">volatile</code>-qualified types, using simple assignments where the left operand is a <code class="code">volatile</code>-qualified non-class type for their value, compound assignments where the left operand is a <code class="code">volatile</code>-qualified non-class type, <code class="code">volatile</code>-qualified function return type, <code class="code">volatile</code>-qualified parameter type, and structured bindings of a <code class="code">volatile</code>-qualified type. This usage was deprecated in C++20. </p> <p>Enabled by default with <samp class="option">-std=c++20</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wzero-as-null-pointer-constant <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wzero-as-null-pointer-constant"> ¶</a></span>
+</dt> <dd>
+<p>Warn when a literal ‘<samp class="samp">0</samp>’ is used as null pointer constant. This can be useful to facilitate the conversion to <code class="code">nullptr</code> in C++11. </p> </dd> <dt>
+ <span><code class="code">-Waligned-new</code><a class="copiable-link" href="#index-Waligned-new"> ¶</a></span>
+</dt> <dd>
+<p>Warn about a new-expression of a type that requires greater alignment than the <code class="code">alignof(std::max_align_t)</code> but uses an allocation function without an explicit alignment parameter. This option is enabled by <samp class="option">-Wall</samp>. </p> <p>Normally this only warns about global allocation functions, but <samp class="option">-Waligned-new=all</samp> also warns about class member allocation functions. </p> </dd> <dt>
+ <span><code class="code">-Wno-placement-new</code><a class="copiable-link" href="#index-Wplacement-new"> ¶</a></span>
+</dt> <dt><code class="code">-Wplacement-new=<var class="var">n</var></code></dt> <dd>
+<p>Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large. </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">char buf [64];
+new (buf) int[64];</pre>
+</div> <p>This warning is enabled by default. </p> <dl class="table"> <dt><code class="code">-Wplacement-new=1</code></dt> <dd>
+<p>This is the default warning level of <samp class="option">-Wplacement-new</samp>. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following <code class="code">new</code> expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array. </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct S { int n, a[1]; };
+S *s = (S *)malloc (sizeof *s + 31 * sizeof s-&gt;a[0]);
+new (s-&gt;a)int [32]();</pre>
+</div> </dd> <dt><code class="code">-Wplacement-new=2</code></dt> <dd>
+<p>At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2. </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct S { int n, a[]; };
+S *s = (S *)malloc (sizeof *s + 32 * sizeof s-&gt;a[0]);
+new (s-&gt;a)int [32]();</pre>
+</div> </dd> </dl> </dd> <dt>
+ <span><code class="code">-Wcatch-value</code><a class="copiable-link" href="#index-Wcatch-value"> ¶</a></span>
+</dt> <dt><code class="code">-Wcatch-value=<var class="var">n</var> <span class="r">(C++ and Objective-C++ only)</span></code></dt> <dd>
+<p>Warn about catch handlers that do not catch via reference. With <samp class="option">-Wcatch-value=1</samp> (or <samp class="option">-Wcatch-value</samp> for short) warn about polymorphic class types that are caught by value. With <samp class="option">-Wcatch-value=2</samp> warn about all class types that are caught by value. With <samp class="option">-Wcatch-value=3</samp> warn about all types that are not caught by reference. <samp class="option">-Wcatch-value</samp> is enabled by <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wconditionally-supported <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wconditionally-supported"> ¶</a></span>
+</dt> <dd>
+<p>Warn for conditionally-supported (C++11 [intro.defs]) constructs. </p> </dd> <dt>
+ <span><code class="code">-Wno-delete-incomplete <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wdelete-incomplete"> ¶</a></span>
+</dt> <dd>
+<p>Do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default. </p> </dd> <dt>
+ <span><code class="code">-Wextra-semi <span class="r">(C++, Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wextra-semi"> ¶</a></span>
+</dt> <dd>
+<p>Warn about redundant semicolons after in-class function definitions. </p> </dd> <dt>
+ <span><code class="code">-Wno-inaccessible-base <span class="r">(C++, Objective-C++ only)</span></code><a class="copiable-link" href="#index-Winaccessible-base"> ¶</a></span>
+</dt> <dd>
+<p>This option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the <samp class="option">-Wextra</samp> option. </p>
+<div class="example smallexample"> <div class="group"><pre class="example-preformatted" data-language="cpp">struct A { int a; };
+
+struct B : A { };
+
+struct C : B, A { };</pre></div>
+</div> </dd> <dt>
+ <span><code class="code">-Wno-inherited-variadic-ctor</code><a class="copiable-link" href="#index-Winherited-variadic-ctor"> ¶</a></span>
+</dt> <dd>
+<p>Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. </p> </dd> <dt>
+ <span><code class="code">-Wno-invalid-offsetof <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wno-invalid-offsetof"> ¶</a></span>
+</dt> <dd>
+<p>Suppress warnings from applying the <code class="code">offsetof</code> macro to a non-POD type. According to the 2014 ISO C++ standard, applying <code class="code">offsetof</code> to a non-standard-layout type is undefined. In existing C++ implementations, however, <code class="code">offsetof</code> typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. </p> <p>The restrictions on <code class="code">offsetof</code> may be relaxed in a future version of the C++ standard. </p> </dd> <dt>
+ <span><code class="code">-Wsized-deallocation <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wsized-deallocation"> ¶</a></span>
+</dt> <dd>
+<p>Warn about a definition of an unsized deallocation function </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void operator delete (void *) noexcept;
+void operator delete[] (void *) noexcept;</pre>
+</div> <p>without a definition of the corresponding sized deallocation function </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void operator delete (void *, std::size_t) noexcept;
+void operator delete[] (void *, std::size_t) noexcept;</pre>
+</div> <p>or vice versa. Enabled by <samp class="option">-Wextra</samp> along with <samp class="option">-fsized-deallocation</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wsuggest-final-types</code><a class="copiable-link" href="#index-Wno-suggest-final-types"> ¶</a></span>
+</dt> <dd>
+<p>Warn about types with virtual methods where code quality would be improved if the type were declared with the C++11 <code class="code">final</code> specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. </p> </dd> <dt>
+ <span><code class="code">-Wsuggest-final-methods</code><a class="copiable-link" href="#index-Wno-suggest-final-methods"> ¶</a></span>
+</dt> <dd>
+<p>Warn about virtual methods where code quality would be improved if the method were declared with the C++11 <code class="code">final</code> specifier, or, if possible, its type were declared in an anonymous namespace or with the <code class="code">final</code> specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of <samp class="option">-Wsuggest-final-types</samp> and then rebuild with new annotations. </p> </dd> <dt>
+ <span><code class="code">-Wsuggest-override</code><a class="copiable-link" href="#index-Wsuggest-override"> ¶</a></span>
+</dt> <dd>
+<p>Warn about overriding virtual functions that are not marked with the <code class="code">override</code> keyword. </p> </dd> <dt>
+ <span><code class="code">-Wuse-after-free</code><a class="copiable-link" href="#index-Wuse-after-free"> ¶</a></span>
+</dt> <dt><code class="code">-Wuse-after-free=<var class="var">n</var></code></dt> <dd>
+<p>Warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without. </p> <dl class="table"> <dt><code class="code">-Wuse-after-free=1</code></dt> <dd><p>At level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call to <code class="code">realloc</code>, regardless of whether or not the call resulted in an actual reallocatio of memory. This includes double-<code class="code">free</code> calls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level. </p></dd> <dt><code class="code">-Wuse-after-free=2</code></dt> <dd>
+<p>At level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to <code class="code">free</code> in the following function is diagnosed at this level: </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct A { int refcount; void *data; };
+
+void release (struct A *p)
+{
+  int refcount = --p-&gt;refcount;
+  free (p);
+  if (refcount == 0)
+    free (p-&gt;data);   // warning: p may be used after free
+}</pre>
+</div> </dd> <dt><code class="code">-Wuse-after-free=3</code></dt> <dd>
+<p>At level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined but equality tests sometimes appear after calls to <code class="code">realloc</code> as an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid legacy code gradually transition to safe alternatives. For example, the equality test in the function below is diagnosed at this level: </p>
+<div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">void adjust_pointers (int**, int);
+
+void grow (int **p, int n)
+{
+  int **q = (int**)realloc (p, n *= 2);
+  if (q == p)
+    return;
+  adjust_pointers ((int**)q, n);
+}</pre>
+</div> <p>To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation. </p>
+</dd> </dl> <p><samp class="option">-Wuse-after-free=2</samp> is included in <samp class="option">-Wall</samp>. </p> </dd> <dt>
+ <span><code class="code">-Wuseless-cast <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wuseless-cast"> ¶</a></span>
+</dt> <dd>
+<p>Warn when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type as that is a way to create a temporary: </p> <div class="example smallexample"> <pre class="example-preformatted" data-language="cpp">struct S { };
+void g (S&amp;&amp;);
+void f (S&amp;&amp; arg)
+{
+  g (S(arg)); // make arg prvalue so that it can bind to S&amp;&amp;
+}</pre>
+</div> </dd> <dt>
+ <span><code class="code">-Wno-conversion-null <span class="r">(C++ and Objective-C++ only)</span></code><a class="copiable-link" href="#index-Wconversion-null"> ¶</a></span>
+</dt> <dd>
+<p>Do not warn for conversions between <code class="code">NULL</code> and non-pointer types. <samp class="option">-Wconversion-null</samp> is enabled by default. </p> </dd> </dl> </div>  <div class="nav-panel"> <p> Next: <a href="objective-c-and-objective-c_002b_002b-dialect-options">Options Controlling Objective-C and Objective-C++ Dialects</a>, Previous: <a href="c-dialect-options">Options Controlling C Dialect</a>, Up: <a href="invoking-gcc">GCC Command Options</a> [<a href="index#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="indices" title="Index" rel="index">Index</a>]</p> </div><div class="_attribution">
+  <p class="_attribution-p">
+    &copy; Free Software Foundation<br>Licensed under the GNU Free Documentation License, Version 1.3.<br>
+    <a href="https://gcc.gnu.org/onlinedocs/gcc-13.1.0/gcc/C_002b_002b-Dialect-Options.html" class="_attribution-link">https://gcc.gnu.org/onlinedocs/gcc-13.1.0/gcc/C_002b_002b-Dialect-Options.html</a>
+  </p>
+</div>