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Showing content from https://en.cppreference.com/w/cpp/language/../language/attributes.html below:

Attribute specifier sequence (since C++11)

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

where attribute-list is a comma-separated sequence of zero or more attribute s (possibly ending with an ellipsis ... indicating a pack expansion)

where attribute-namespace is an identifier and argument-list is a sequence of tokens where parentheses, brackets and braces are balanced (balanced-token-seq).

1) Simple attribute, such as [[noreturn]].

2) Attribute with a namespace, such as [[gnu::unused]].

3) Attribute with arguments, such as [[deprecated("because")]].

4) Attribute with both a namespace and an argument list.

If using namespace: appears in the beginning of an attribute list, no other attributes in the attribute list can specify a namespace: the namespace specified in a using applies to them all:

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

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 an if, and not with a class declaration. [[omp::parallel()]] could be an attribute that applies to a code block or to a for loop, but not to the type int, 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.

The 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).

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[[] { return 0; }()] = 1;  // error
    int 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)

An attribute without attribute-namespace and an attribute-namespace whose name is either std or std followed by one or more digits is reserved for future standardization. That is, every non-standard attribute is in the attribute-namespace provided by the implementation, e.g. [[gnu::may_alias]], [[clang::trivial_abi]], and [[msvc::noop_dtor]].

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 be ODR-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.

(C++11)(removed in C++26)

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

[[gnu::always_inline]] [[gnu::hot]] [[gnu::const]] [[nodiscard]]
inline int 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() { return 0; }
 
int main() {}

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

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