9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.8 Maybe unused attribute [dcl.attr.unused]

1
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The attribute-token maybe_unused indicates that a name, label, or entity is possibly intentionally unused.
No attribute-argument-clause shall be present.
2
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The attribute may be applied to the declaration of a class, typedef-name, variable (including a structured binding declaration), non-static data member, function, enumeration, or enumerator, or to an identifier label ([stmt.label]).
3
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A name or entity declared without the maybe_unused attribute can later be redeclared with the attribute and vice versa.
An entity is considered marked after the first declaration that marks it.
4
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Recommended practice: For an entity marked maybe_unused, implementations should not emit a warning that the entity or its structured bindings (if any) are used or unused.
For a structured binding declaration not marked maybe_unused, implementations should not emit such a warning unless all of its structured bindings are unused.
For a label to which maybe_unused is applied, implementations should not emit a warning that the label is used or unused.
The value of a has-attribute-expression for the maybe_unused attribute should be 0 if the attribute does not cause suppression of such warnings.
5
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[Example 1: [[maybe_unused]] void f([[maybe_unused]] bool thing1, [[maybe_unused]] bool thing2) { [[maybe_unused]] bool b = thing1 && thing2; assert(b); #ifdef NDEBUG goto x; #endif [[maybe_unused]] x: }
Implementations should not warn that b or x is unused, whether or not NDEBUG is defined.
— end example]