11 Classes [class]

11.3 Class names [class.name]

A class definition introduces a new type.

[Example 1:

struct X { int a; }; struct Y { int a; }; X a1; Y a2; int a3; declares three variables of three different types.

This implies that a1 = a2; // error: Y assigned to X a1 = a3; // error: int assigned to X are type mismatches, and that int f(X); int f(Y); declare overloads ([over]) named f and not simply a single function f twice.

For the same reason, struct S { int a; }; struct S { int a; }; // error: double definition is ill-formed because it defines S twice.

— end example]

[Note 1:

It can be necessary to use an elaborated-type-specifier 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]).

[Example 2: struct stat { // ... }; stat gstat; // use plain stat to define variable int stat(struct stat*); // stat now also names a function void f() { struct stat* ps; // struct prefix needed to name struct stat stat(ps); // call stat function } — end example]

An elaborated-type-specifier can also be used to declare an identifier as a class-name.

[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 s struct s { char* p; }; // define local struct s struct s; // redeclaration, has no effect } — end example]

Such declarations allow definition of classes that refer to each other.

[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], operator functions in [over.oper].

— end example]

— end note]

[Note 2:

An elaborated-type-specifier ([dcl.type.elab]) can also be used as a type-specifier as part of a declaration.

It differs from a class declaration in that it can refer to an existing class of the given name.

— end note]

[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]

[Note 3:

The declaration of a class name takes effect immediately after the identifier is seen in the class definition or elaborated-type-specifier.

For example, 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.

This means that the elaborated form class A must be used to refer to the class.

Such artistry with names can be confusing and is best avoided.

— end note]

A simple-template-id is only a class-name if its template-name names a class template.