The attribute-token nodiscard may be applied to the declarator-id in a function declaration or to the declaration of a class or enumeration. It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
[ Note: A nodiscard call is a function call expression that calls a function previously declared nodiscard, or whose return type is a possibly cv-qualified class or enumeration type marked nodiscard. Appearance of a nodiscard call as a potentially-evaluated discarded-value expression (Clause [expr]) is discouraged unless explicitly cast to void. Implementations are encouraged to issue a warning in such cases. This is typically because discarding the return value of a nodiscard call has surprising consequences. — end note ]
[ Example:
struct [[nodiscard]] error_info { /* ... */ };
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
enable_missile_safety_mode(); // warning encouraged
launch_missiles();
}
error_info &foo();
void f() { foo(); } // warning not encouraged: not a nodiscard call, because neither
// the (reference) return type nor the function is declared nodiscard
— end example ]