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

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Attributes
(C++14)
(C++17)
maybe_unused
(C++17)
(C++20)(C++20)
(C++23)
 

Suppresses warnings on unused entities.

Contents

[edit] Syntax

[[maybe_unused]]

[edit] Explanation

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

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.

For labels declared [[maybe_unused]], if they are unused, the warning on unused labels issued by the compiler is suppressed.

(since C++26)

[edit] 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() {}

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
CWG 2360 C++17 could not apply [[maybe_unused]] to structured bindings allowed

[edit] See also

C documentation for maybe_unused