[class.name]

# 11 Classes [[class]](./#class)

## 11.3 Class names [class.name]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L408)

A class definition introduces a new type[.](#1.sentence-1)

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

struct X { int a; };struct Y { int a; };
X a1;
Y a2;int a3; declares three variables of three different types[.](#1.sentence-2)

This implies thata1 = a2; // error: Y assigned to X a1 = a3; // error: int assigned to X are type mismatches, and thatint f(X);int f(Y);declare overloads ([[over]](over "12 Overloading")) named f and not
simply a single function f twice[.](#1.sentence-3)

For the same reason,struct S { int a; };struct S { int a; }; // error: double definition is ill-formed because it defines S twice[.](#1.sentence-4)

— *end example*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L438)

[*Note [1](#note-1)*:

It can be necessary to use an [*elaborated-type-specifier*](dcl.type.elab#nt:elaborated-type-specifier "9.2.9.5 Elaborated type specifiers [dcl.type.elab]") to refer to a class
that belongs to a scope in which its name is also bound to
a variable, function, or enumerator ([[basic.lookup.elab]](basic.lookup.elab "6.5.6 Elaborated type specifiers"))[.](#2.sentence-1)

[*Example [2](#example-2)*: struct stat {// ...};

stat gstat; // use plain stat to define variableint stat(struct stat*); // stat now also names a functionvoid f() {struct stat* ps; // struct prefix needed to name struct stat stat(ps); // call stat function} — *end example*]

An [*elaborated-type-specifier*](dcl.type.elab#nt:elaborated-type-specifier "9.2.9.5 Elaborated type specifiers [dcl.type.elab]") can also be used to declare
an [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") as a [*class-name*](class.pre#nt:class-name "11.1 Preamble [class.pre]")[.](#2.sentence-2)

[*Example [3](#example-3)*: struct s { int a; };

void g() {struct s; // hide global struct s with a block-scope declaration s* p; // refer to local struct sstruct s { char* p; }; // define local struct sstruct s; // redeclaration, has no effect} — *end example*]

Such declarations allow definition of classes that refer to each other[.](#2.sentence-3)

[*Example [4](#example-4)*: class Vector;

class Matrix {// ...friend Vector operator*(const Matrix&, const Vector&);};

class Vector {// ...friend Vector operator*(const Matrix&, const Vector&);};

Declaration of friends is described in [[class.friend]](class.friend "11.8.4 Friends"),
operator functions in [[over.oper]](over.oper "12.4 Overloaded operators")[.](#2.sentence-4)

— *end example*]

— *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L497)

[*Note [2](#note-2)*:

An [*elaborated-type-specifier*](dcl.type.elab#nt:elaborated-type-specifier "9.2.9.5 Elaborated type specifiers [dcl.type.elab]") ([[dcl.type.elab]](dcl.type.elab "9.2.9.5 Elaborated type specifiers")) can also
be used as a [*type-specifier*](dcl.type.general#nt:type-specifier "9.2.9.1 General [dcl.type.general]") as part of a declaration[.](#3.sentence-1)

It
differs from a class declaration in that it can refer to
an existing class of the given name[.](#3.sentence-2)

— *end note*]

[*Example [5](#example-5)*: struct s { int a; };

void g(int s) {struct s* p = new struct s; // global s p->a = s; // parameter s} — *end example*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L517)

[*Note [3](#note-3)*:

The declaration of a class name takes effect immediately after the[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") is seen in the class definition or[*elaborated-type-specifier*](dcl.type.elab#nt:elaborated-type-specifier "9.2.9.5 Elaborated type specifiers [dcl.type.elab]")[.](#4.sentence-1)

[*Example [6](#example-6)*:

class A * A; first specifies A to be the name of a class and then redefines
it as the name of a pointer to an object of that class[.](#4.sentence-2)

This means that
the elaborated form class A must be used to refer to the
class[.](#4.sentence-3)

Such artistry with names can be confusing and is best avoided[.](#4.sentence-4)

— *end example*]

— *end note*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L534)

A [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]") is only a [*class-name*](class.pre#nt:class-name "11.1 Preamble [class.pre]") if its [*template-name*](temp.names#nt:template-name "13.3 Names of template specializations [temp.names]") names a class template[.](#5.sentence-1)