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]], 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]] 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.

See also