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

Suppresses warnings on unused entities.

Syntax

Explanation

This attribute can appear in the declaration of the following entities:

• class : struct [ [ maybe_unused ] ] S ;
• typedef , including those declared by alias declaration : [ [ maybe_unused ] ] typedef S * PS ; , using PS [ [ maybe_unused ] ] = S * ;
• variable, including static data member : [ [ maybe_unused ] ] int x ;
• non-static data member : union U { [ [ maybe_unused ] ] int n ; } ; ,
• function : [ [ maybe_unused ] ] void f ( ) ;
• enumeration : enum [ [ maybe_unused ] ] E { } ;
• enumerator: enum { A [ [ maybe_unused ] ] , B [ [ maybe_unused ] ] = 42 } ;
• structured binding : [ [ maybe_unused ] ] auto [ a, b ] = std:: make_pair ( 42 , 0.23 ) ;

For entities declared [ [ maybe_unused ] ] , if the entities or their structured bindings are unused, the warning on unused entities issued by the compiler is suppressed.

Example

#include <cassert>
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2)
{
    [[maybe_unused]] lbl: // the label “lbl” is not used, no warning
    [[maybe_unused]] bool b = not false and not true;
    assert(b); // in release mode, assert is compiled out, and “b” is unused
               // no warning because it is declared [[maybe_unused]]
} // parameters “thing1” and “thing2” are not used, no warning
int main() {}

Defect reports

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

References

See also