C attribute: nodiscard (since C23)
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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.
Syntax
[[ nodiscard ]][[ __nodiscard__ ]]
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(1) | ||||||||
[[ nodiscard ( string-literal ) ]][[ __nodiscard__ ( string-literal ) ]]
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(2) | ||||||||
| string-literal | - | text that could be used to explain the rationale for why the result should not be discarded |
Explanation
Appears in a function declaration, enumeration declaration, or struct/union declaration.
If, from a discarded-value expression other than a cast to void,
- a function declared
nodiscardis called, or - a function returning a struct/union/enum declared
nodiscardis called,
the compiler is encouraged to issue a warning.
The string-literal, if specified, is usually included in the warnings.
Example
Run this code
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
See also
| C++ documentation for nodiscard
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