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Showing content from http://en.cppreference.com/w/cpp/../cpp/../cpp/../c/language/../language/attributes/nodiscard.html below:

C attribute: nodiscard (since C23)

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.

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 nodiscard is called, or
• a function returning a struct/union/enum declared 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

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