If a function declarednodiscard or a function returning a struct/union/enum declarednodiscard by value is called from adiscarded-value expression other than a cast tovoid, the compiler is encouraged to issue a warning.
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[[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 adiscarded-value expression other than a cast tovoid,
nodiscard is called, ornodiscard is called,the compiler is encouraged to issue a warning.
Thestring-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(){staticstruct 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 valueint z= strategic_value(0,0);// OK: return value is not discardedreturn z;}
Possible output:
game.cpp:5:4: warning: ignoring return value of function declared with 'nodiscard' attributegame.cpp:17:5: warning: ignoring return value of function declared with 'nodiscard' attribute: PURE FUN
C++ documentation fornodiscard |
