C++ attribute: nodiscard (since C++17)

If a function declared nodiscard or a function returning an enumeration or class 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

Explanation

Appears in a function declaration, enumeration declaration, or class declaration.

If, from a discarded-value expression other than a cast to void ,

• a function declared nodiscard is called, or
• a function returning an enumeration or class declared nodiscard by value is called, or
• a constructor declared nodiscard is called by explicit type conversion or static_cast , or
• an object of an enumeration or class type declared nodiscard is initialized by explicit type conversion or static_cast ,

the compiler is encouraged to issue a warning.

Example

struct [[nodiscard]] error_info { /*...*/ };
error_info enable_missile_safety_mode() { /*...*/ return {}; }
void launch_missiles() { /*...*/ }
void test_missiles()
{
    enable_missile_safety_mode(); // compiler may warn on discarding a nodiscard value
    launch_missiles();
}
error_info& foo() { static error_info e; /*...*/ return e; }
void f1() { foo(); } // nodiscard type is not returned by value, no warning
// nodiscard( string-literal ) (since C++20):
[[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
    auto z = strategic_value(0, 0); // OK: return value is not discarded
    return z;
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

References

See also