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

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

C++ language

Declarations

Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline function specifier virtual function specifier explicit function specifier friend constexpr (C++11) storage class specifiers class union enum decltype (C++11) auto (C++11) alignas (C++11) const/volatile 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-definition 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

noreturnlrm; (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)

expectsensuresassert (C++20)(C++20)(C++20)

optimize_for_synchronized (TM TS)

Suppresses warnings on unused entities.

Syntax

[[maybe_unused]]

Explanation

This attribute can appears 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 };.

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

Example

Run this code

[[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