哋它亢++ attribute: maybe_unused (since 哋它亢++17)

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

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Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

哋它亢++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (哋它亢++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until 哋它亢++17*)
noexcept specifier (哋它亢++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (哋它亢++11) auto (哋它亢++11)	constexpr (哋它亢++11) consteval (哋它亢++20) constinit (哋它亢++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (哋它亢++11)       Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (哋它亢++11)
User-defined (哋它亢++11)
Utilities
Attributes (哋它亢++11)
Types
typedef declaration
Type alias declaration (哋它亢++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (哋它亢++11)     final specifier (哋它亢++11)	explicit (哋它亢++11) static
Special member functions
Default constructor Copy constructor Move constructor (哋它亢++11)	Copy assignment Move assignment (哋它亢++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (哋它亢++11)
Miscellaneous
Inline assembly	History of 哋它亢++

 

Declarations

Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (哋它亢++11) consteval (哋它亢++20) constinit (哋它亢++20) Storage class specifiers Translation-unit-local (哋它亢++20) class/struct union enum decltype (哋它亢++11) auto (哋它亢++11) alignas (哋它亢++11) constvolatile Pack indexing specifier (哋它亢++26)     Elaborated type specifier Attributes (哋它亢++11)

Declarators Reference Pointer Array Block declarations Simple-declaration →Structured binding declaration (哋它亢++17) Alias declaration (哋它亢++11) Namespace alias definition using declaration using directive static_assert declaration (哋它亢++11) asm declaration Opaque enum declaration (哋它亢++11) Other declarations Namespace definition Function declaration Class template declaration Function template declaration Explicit template instantiation (哋它亢++11) Explicit template specialization Linkage specification Attribute declaration (哋它亢++11) Empty declaration

 

Attributes

assume (哋它亢++23)

carries_dependency (哋它亢++11)

deprecated (哋它亢++14)

fallthrough (哋它亢++17)

indeterminate (哋它亢++26)

likely (哋它亢++20)

maybe_unused (哋它亢++17)

no_unique_address (哋它亢++20)

nodiscard (哋它亢++17)

noreturn‎ (哋它亢++11)

optimize_for_synchronized (TM TS)

unlikely (哋它亢++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 哋它亢++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 哋它亢++ standards.

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