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

 

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Attributes

assume (C++23)

carries_dependency (C++11)

deprecated (C++14)

fallthrough (C++17)

indeterminate (C++26)

likely (C++20)

maybe_unused (C++17)

nodiscard (C++17)

noreturn (C++11)

no_unique_address (C++20)

optimize_for_synchronized (TM TS)

unlikely (C++20)

 

Suppresses warnings on unused entities.

Syntax

[[maybe_unused]]

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.

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

(since C++26)

Example

Run this code

#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.

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

References

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

• 10.6.6 Maybe unused attribute [dcl.attr.unused]

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

• 9.12.7 Maybe unused attribute [dcl.attr.unused]

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

• 9.12.8 Maybe unused attribute [dcl.attr.unused]

See also

C documentation for maybe_unused