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attribute specifier sequence(since C++11)

From cppreference.com
< cpp‎ | language

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

[[attr]] [[attr1, attr2, attr3(args)]] [[namespace::attr(args)]] alignas_specifier

[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: [[probably(true)]] can only be used with an if, and not with an class declaration. [[omp::parallel()]] can apply to a code block or to a for loop, but not to the type int, etc.

In declarations, attributes may appear both before 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.

alignas_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 where alignas is permitted)

[edit] Standard attributes

Only the following attributes are defined by the C++ standard. All other attributes are implementation-specific.

[[noreturn]] Indicates that the function does not return.
This attribute applies to function declarations only. The behavior is undefined if the function with this attribute actually returns.
The following standard functions have this attribute: std::_Exit, std::abort, std::exit, std::quick_exit, std::unexpected, std::terminate, std::rethrow_exception, std::throw_with_nested, std::nested_exception::rethrow_nested
[[carries_dependency]] Indicates that dependency chain in release-consume std::memory_order propagates in and out of the function, which allows the compiler to skip unnecessary memory fence instructions.
This attribute may appear in two situations:

1) it may apply to the parameter declarations of a function or lambda-expressions, in which case it indicates that initialization of the parameter carries dependency into lvalue-to-rvalue conversion of that object.
2) It may apply to the function declaration as a whole, in which case it indicates that the return value carries dependency to the evaluation of the function call expression.

[[deprecated]](C++14)
[[deprecated("reason")]](C++14)
Indicates that the use of the name or entity declared with this attribute is allowed, but discouraged for some reason. This attribute is allowed in declarations of classes, typedef-names, variables, non-static data members, functions, enumerations, and template specializations. A name declared non-deprecated may be redeclared deprecated. A name declared deprecated cannot be un-deprecated by redeclaring it without this attribute.

[edit] Example