[dcl.attr.annotation]

# 9 Declarations [[dcl]](./#dcl)

## 9.13 Attributes [[dcl.attr]](dcl.attr#annotation)

### 9.13.12 Annotations [dcl.attr.annotation]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L10191)

An annotation may be applied to any declaration of a
type,
type alias,
variable,
function,
namespace,
enumerator,[*base-specifier*](class.derived.general#nt:base-specifier "11.7.1 General [class.derived.general]"), or
non-static data member[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L10202)

Let E be the expressionstd​::​meta​::​reflect_constant([*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]"))[.](#2.sentence-1)

E shall be a constant expression;
the result of E is the [*underlying constant*](#def:constant,underlying "9.13.12 Annotations [dcl.attr.annotation]") of the annotation[.](#2.sentence-2)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L10208)

Each [*annotation*](dcl.attr.grammar#nt:annotation "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") produces a unique annotation[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L10211)

Substituting into an [*annotation*](dcl.attr.grammar#nt:annotation "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") is not in the immediate context[.](#4.sentence-1)

[*Example [1](#example-1)*:

[[=1]] void f();[[=2, =3, =2]] void g();void g [[=4, =2]] ();f has one annotation
and g has five annotations[.](#4.sentence-2)

These can be queried with metafunctions
such as std​::​​meta​::​​annotations_of ([[meta.reflection.annotation]](meta.reflection.annotation "21.4.18 Annotation reflection"))[.](#4.sentence-3)

— *end example*]

[*Example [2](#example-2)*: template<class T>[[=T::type()]] void f(T t);

void f(int);

void g() { f(0); // OK f('0'); // error, substituting into the annotation results in an invalid expression} — *end example*]