Introduces implementation-defined attributes for types, objects, code, etc.

[edit]Syntax

whereattribute-list is a comma-separated sequence of zero or moreattribute s (possibly ending with an ellipsis... indicating apack expansion)

whereattribute-namespace is anidentifier andargument-list is a sequence of tokens where parentheses, brackets and braces are balanced (balanced-token-seq).

[[using CC: opt(1), debug]]// same as [[CC::opt(1), CC::debug]][[using CC: CC::opt(1)]]// error: cannot combine using and scoped attribute

[edit]Explanation

Attributes provide the unified standard syntax for implementation-defined language extensions, such as the GNU and IBM language extensions__attribute__((...)), Microsoft extension__declspec(), etc.

An attribute can be used almost everywhere in the C++ program, and can be applied to almost everything: to types, to variables, to functions, to names, to code blocks, to entire translation units, although each particular attribute is only valid where it is permitted by the implementation:[[expect_true]] could be an attribute that can only be used with anif, and not with a class declaration.[[omp::parallel()]] could be an attribute that applies to a code block or to afor loop, but not to the typeint, etc (note these two attributes are fictional examples, see below for the standard and some non-standard attributes).

In declarations, attributes may appear both before the whole declaration and directly after the name of the entity that is declared, in which case they are combined. In most other situations, attributes apply to the directly preceding entity.

Thealignas specifier is a part of the attribute specifier sequence, although it has different syntax. It may appear where the[[...]] attributes appear and may mix with them (provided it is used wherealignas is permitted).

Two consecutive left square bracket tokens ([[) may only appear when introducing an attribute-specifier or inside an attribute argument.

void f(){int y[3];    y[[]{return0;}()]=1;// errorint i[[cats::meow([[]])]];// OK}

Besides the standard attributes listed below, implementations may support arbitrary non-standard attributes with implementation-defined behavior.All attributes unknown to an implementation are ignored without causing an error.(since C++17)

[edit]Standard attributes

The following attributes are defined by the C++ standard.

Standard attributes cannot be syntactically ignored: they cannot contain syntax errors, must be applied to the correct target, and entities in the arguments must beODR-use.

Standard attributes cannot be semantically ignored either: the behavior with all instances of a particular standard attribute removed would have been a conforming behavior for the original program with the attribute present.

[edit]Notes

The presence of each individual attribute on a given platform can be checked with__has_cpp_attribute preprocessor macro.

[edit]Example

[[gnu::always_inline]][[gnu::hot]][[gnu::const]][[nodiscard]]inlineint f();// declare f with four attributes [[gnu::always_inline, gnu::const, gnu::hot, nodiscard]]int f();// same as above, but uses a single attr specifier that contains four attributes // C++17:[[using gnu:const, always_inline, hot]][[nodiscard]]int f[[gnu::always_inline]]();// an attribute may appear in multiple specifiers int f(){return0;} int main(){}

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

[edit]External links

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