C attribute: maybe_unused (since C23)

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Suppresses warnings on unused entities.


[edit] Syntax

[[ maybe_unused ]]
[[ __maybe_unused__ ]]

[edit] Explanation

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

  • struct/union: struct [[maybe_unused]] S;,
  • typedef name: [[maybe_unused]] typedef S* PS;,
  • object: [[maybe_unused]] int x;,
  • struct/union member: union U { [[maybe_unused]] int n; };,
  • function: [[maybe_unused]] void f(void);,
  • enumeration: enum [[maybe_unused]] E {};,
  • enumerator: enum { A [[maybe_unused]], B [[maybe_unused]] = 42 };.

If the compiler issues warnings on unused entities, that warning is suppressed for any entity declared maybe_unused.

[edit] Example

#include <assert.h>
[[maybe_unused]] void f([[maybe_unused]] _Bool cond1, [[maybe_unused]] _Bool cond2)
   [[maybe_unused]] _Bool b = cond1 && cond2;
   assert(b); // in release mode, assert is compiled out, and b is unused
              // no warning because it is declared [[maybe_unused]]
} // parameters cond1 and cond2 are not used, no warning
int main(void)
    f(1, 1);

[edit] See also

C++ documentation for maybe_unused