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<h1 id="firstHeading" class="firstHeading">Static assertion <span class="t-mark-rev t-since-c11">(since C11)</span>
</h1> <h3 id="Syntax"> Syntax</h3> <table class="t-sdsc-begin"> <tr class="t-sdsc"> <td> <code>_Static_assert</code> <code>(</code> <span class="t-spar">expression</span> <code>,</code> <span class="t-spar">message</span> <code>)</code> </td> <td class="t-sdsc-nopad"> </td> <td> <span class="t-mark-rev t-since-c11">(since C11)</span><span class="t-mark-rev t-deprecated-c23">(deprecated in C23)</span> </td>
</tr> <tr class="t-sdsc"> <td> <code>static_assert</code> <code>(</code> <span class="t-spar">expression</span> <code>,</code> <span class="t-spar">message</span> <code>)</code> </td> <td class="t-sdsc-nopad"> </td> <td> <span class="t-mark-rev t-since-c23">(since C23)</span> </td>
</tr> <tr class="t-sdsc"> <td> <code>_Static_assert</code> <code>(</code> <span class="t-spar">expression</span> <code>)</code> </td> <td class="t-sdsc-nopad"> </td> <td> <span class="t-mark-rev t-since-c23">(since C23)</span><span class="t-mark-rev t-deprecated-c23">(deprecated in C23)</span> </td>
</tr> <tr class="t-sdsc"> <td> <code>static_assert</code> <code>(</code> <span class="t-spar">expression</span> <code>)</code> </td> <td class="t-sdsc-nopad"> </td> <td> <span class="t-mark-rev t-since-c23">(since C23)</span> </td>
</tr>
</table> <table class="t-par-begin"> <tr class="t-par"> <td> <span class="t-spar">expression</span> </td> <td> - </td> <td> any <a href="constant_expression" title="c/language/constant expression">integer constant expression</a> </td>
</tr> <tr class="t-par"> <td> <span class="t-spar">message</span> </td> <td> - </td> <td> any <a href="string_literal" title="c/language/string literal">string literal</a> </td>
</tr>
</table> <table class="t-rev-begin"> <tr class="t-rev t-until-c23">
<td> <p>This keyword is also available as convenience macro <a href="../error/static_assert" title="c/error/static assert"><code>static_assert</code></a>, available in the header <a href="../error" title="c/error"><code><assert.h></code></a>.</p>
</td> <td><span class="t-mark-rev t-until-c23">(until C23)</span></td>
</tr> <tr class="t-rev t-since-c23">
<td> <p>Both of <code>static_assert</code> and <code>_Static_assert</code> have the same effects. <code>_Static_assert</code> is a deprecated spelling that is kept for compatibility.</p>
<p>An implementation may also define <code>static_assert</code> and/or <code>_Static_assert</code> as predefined macros, and <code>static_assert</code> is no longer provided by <a href="../error" title="c/error"><code><assert.h></code></a>.</p>
</td> <td><span class="t-mark-rev t-since-c23">(since C23)</span></td>
</tr> </table> <h3 id="Explanation"> Explanation</h3> <p>The constant expression is evaluated at compile time and compared to zero. If it compares equal to zero, a compile-time error occurs and the compiler <span class="t-rev-inl t-until-c23"><span>must display <span class="t-spar">message</span> as part of the error message (except that characters not in <a href="charset" title="c/language/charset">basic character set</a> are not required to be displayed)</span><span><span class="t-mark-rev t-until-c23">(until C23)</span></span></span><span class="t-rev-inl t-since-c23"><span>should display <span class="t-spar">message</span> (if provided) as part of the error message</span><span><span class="t-mark-rev t-since-c23">(since C23)</span></span></span>.</p>
<p>Otherwise, if <span class="t-spar">expression</span> does not equal zero, nothing happens; no code is emitted.</p>
<h3 id="Keywords"> Keywords</h3> <p><a href="../keyword/_static_assert" title="c/keyword/ Static assert"><code>_Static_assert</code></a>, <a href="../keyword/static_assert" title="c/keyword/static assert"><code>static_assert</code></a></p>
<h3 id="Example"> Example</h3> <div class="t-example"> <div class="c source-c"><pre data-language="c">#include <assert.h> // no longer needed since C23
int main(void)
{
// Test if math works, C23:
static_assert((2 + 2) % 3 == 1, "Whoa dude, you knew!");
// Pre-C23 alternative:
_Static_assert(2 + 2 * 2 == 6, "Lucky guess!?");
// This will produce an error at compile time.
// static_assert(sizeof(int) < sizeof(char), "Unmet condition!");
constexpr int _42 = 2 * 3 * 2 * 3 + 2 * 3;
static_assert(_42 == 42); // the message string can be omitted.
// const int _13 = 13;
// Compile time error - not an integer constant expression:
// static_assert(_13 == 13);
}</pre></div> </div> <h3 id="References"> References</h3> <ul>
<li> C23 standard (ISO/IEC 9899:2023): </li>
<ul><li> 6.7.11 Static assertions (p: TBD) </li></ul>
<li> C17 standard (ISO/IEC 9899:2018): </li>
<ul>
<li> 6.7.10 Static assertions (p: 105) </li>
<li> 7.2 Diagnostics <assert.h> (p: 135) </li>
</ul>
<li> C11 standard (ISO/IEC 9899:2011): </li>
<ul>
<li> 6.7.10 Static assertions (p: 145) </li>
<li> 7.2 Diagnostics <assert.h> (p: 186-187) </li>
</ul>
</ul> <h3 id="See_also"> See also</h3> <table class="t-dsc-begin"> <tr class="t-dsc"> <td> <div><a href="../error/assert" title="c/error/assert"> <span class="t-lines"><span>assert</span></span></a></div> </td> <td> aborts the program if the user-specified condition is not <code>true</code>. May be disabled for release builds <br> <span class="t-mark">(function macro)</span> </td>
</tr> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/language/static_assert" title="cpp/language/static assert">C++ documentation</a></span> for <span class=""><span><code>static_assert</code> declaration</span></span> </td>
</tr> </table> <div class="_attribution">
<p class="_attribution-p">
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