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

Suppresses warnings on unused entities.

[edit] Syntax

[edit] 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.

[edit] 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() {}

[edit] Defect reports

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

[edit] References

[edit] See also

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