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Showing content from http://en.cppreference.com/w/cpp/algorithm/../language/../language/attributes/maybe_unused.html below:

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);

result binding: post(r [[maybe_unused]] : this->empty())

(since C++26)

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]] lbl: // the label âlblâ is not used, no warning
    [[maybe_unused]] bool b = not false and not true;
    assert(b); // in release mode, assert is compiled out, and âbâ is unused
               // no warning because it is declared [[maybe_unused]]
} // parameters âthing1â and âthing2â 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] References

C++23 standard (ISO/IEC 14882:2024):

9.12.8 Maybe unused attribute [dcl.attr.unused]

C++20 standard (ISO/IEC 14882:2020):

9.12.7 Maybe unused attribute [dcl.attr.unused]

C++17 standard (ISO/IEC 14882:2017):

10.6.6 Maybe unused attribute [dcl.attr.unused]

[edit] See also

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