attribute specifier sequence(since C++11)

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

Formally, the syntax is.

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

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

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 a 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).

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).

Standard attributes

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

Example

#include <cassert>
[[ noreturn ]] void f() {
  throw "error";
  // OK
}
 
[[ noreturn ]] void q(int i) {
  // behavior is undefined if called with an argument <= 0
  if (i > 0) {
    throw "positive";
  }
}
 
void f(int n) {
  void g(), h(), i();
  switch (n) {
    case 1:
    case 2:
      g();
     [[fallthrough]];
    case 3: // no warning on fallthrough
      h();
    case 4: // compiler may warn on fallthrough
      i();
      [[fallthrough]]; // ill­formed, not before a case label
  }
}
 
struct [[nodiscard]] error_info { };
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
   enable_missile_safety_mode(); // compiler may warn on discarding a nodiscard value
   launch_missiles();
}
error_info& foo();
void f1() {
    foo(); // nodiscard type is not returned by value, no warning
} 
 
 
[[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

External links

C++ attributes supported by Clang.

See also