If a function declared nodiscard or a function returning a struct/union/enum declared nodiscard by value is called from a discarded-value expression other than a cast to void, the compiler is encouraged to issue a warning.
[[ nodiscard ]][[ __nodiscard__ ]] | (1) | |
[[ nodiscard ( string-literal ) ]][[ __nodiscard__ ( string-literal ) ]] | (2) |
| string-literal | - | text that could be used to explain the rationale for why the result should not be discarded |
Appears in a function declaration, enumeration declaration, or struct/union declaration.
If, from a discarded-value expression other than a cast to void,
nodiscard is called, or nodiscard is called, the compiler is encouraged to issue a warning.
The string-literal, if specified, is usually included in the warnings.
struct [[nodiscard]] error_info { int status; /*...*/ };
struct error_info enable_missile_safety_mode() { /*...*/ return (struct error_info){0}; }
void launch_missiles() { /*...*/ }
void test_missiles() {
enable_missile_safety_mode(); // compiler may warn on discarding a nodiscard value
launch_missiles();
}
struct error_info* foo() { static struct error_info e; /*...*/ return &e; }
void f1() {
foo(); // nodiscard type itself is not returned, no warning
}
// nodiscard( string-literal ):
[[nodiscard("PURE FUN")]] int strategic_value(int x, int y) { return x ^ y; }
int main()
{
strategic_value(4,2); // compiler may warn on discarding a nodiscard value
int z = strategic_value(0,0); // OK: return value is not discarded
return z;
}Possible output:
game.cpp:5:4: warning: ignoring return value of function declared with 'nodiscard' attribute game.cpp:17:5: warning: ignoring return value of function declared with 'nodiscard' attribute: PURE FUN
C++ documentation for nodiscard |
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https://en.cppreference.com/w/c/language/attributes/nodiscard