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/union: 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]]

Example

#include <cassert>
 
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2)
{
    [[maybe_unused]] lb: // the label “lb” is not used, no warning
    [[maybe_unused]] bool b = thing1 && thing2;
    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