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

 

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Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline virtual function specifier explicit function specifier    friend constexpr (C++11) consteval (C++20) constinit (C++20) storage class specifiers translation-unit-local (C++20) class/struct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile elaborated type specifier attributes (C++11)

Declarators reference pointer array Block declarations simple-declaration → structured binding declaration (C++17) alias declaration(C++11) namespace alias definition using-declaration using-directive static_assert declaration (C++11) asm-declaration opaque enum declaration(C++11) Other declarations namespace definition function declaration class template declaration function template declaration explicit template instantiation(C++11) explicit template specialization linkage specification attribute declaration (C++11) empty declaration

 

Attributes

noreturn‎ (C++11)

carries_dependency (C++11)

deprecated (C++14)

fallthrough (C++17)

nodiscard (C++17)

maybe_unused (C++17)

likelyunlikely (C++20)(C++20)

no_unique_address (C++20)

optimize_for_synchronized (TM TS)

 

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 entites declared [[maybe_unused]], if the entities or their structured bindings are unused, the warning on unused entities issued by the compiler is suppressed.

Example

Run this code

#include <cassert>
 
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2)
{
    [[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