[temp.class.general]

# 13 Templates [[temp]](./#temp)

## 13.7 Template declarations [[temp.decls]](temp.decls#temp.class.general)

### 13.7.2 Class templates [[temp.class]](temp.class#general)

#### 13.7.2.1 General [temp.class.general]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L2650)

A[*class template*](#def:template,class "13.7.2.1 General [temp.class.general]") defines the layout and operations
for an unbounded set of related types[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L2656)

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

It is possible for a single class templateList to provide an unbounded set of class definitions:
one class List<T> for every type T,
each describing a linked list of elements of type T[.](#2.sentence-1)

Similarly, a class template Array describing a contiguous,
dynamic array can be defined like this:template<class T> class Array { T* v; int sz;public:explicit Array(int);
 T& operator[](int);
 T& elem(int i) { return v[i]; }};

The prefix template<class T> specifies that a template is being declared and that a[*type-name*](dcl.type.simple#nt:type-name "9.2.9.3 Simple type specifiers [dcl.type.simple]") T can be used in the declaration[.](#2.sentence-3)

In other words,Array is a parameterized type withT as its parameter[.](#2.sentence-4)

â *end example*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L2686)

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

When a member of a class
template is defined outside of the class template definition,
the member definition is defined as a template definition with the[*template-head*](temp.pre#nt:template-head "13.1 Preamble [temp.pre]") equivalent to that
of the class template[.](#3.sentence-1)

The names of the template parameters used in the definition of the member
can differ from the template parameter names used in the class
template definition[.](#3.sentence-2)

The class template name in the member definition is followed by
the template argument list of the [*template-head*](temp.pre#nt:template-head "13.1 Preamble [temp.pre]") ([[temp.arg]](temp.arg "13.4 Template arguments"))[.](#3.sentence-3)

[*Example [2](#example-2)*: template<class T1, class T2> struct A {void f1(); void f2();};

template<class T2, class T1> void A<T2,T1>::f1() { } // OKtemplate<class T2, class T1> void A<T1,T2>::f2() { } // error

template<class ... Types> struct B {void f3(); void f4();};

template<class ... Types> void B<Types ...>::f3() { } // OKtemplate<class ... Types> void B<Types>::f4() { } // error

template<typename T> concept C = true;template<typename T> concept D = true;

template<C T> struct S {void f(); void g(); void h(); template<D U> struct Inner;};

template<C A> void S<A>::f() { } // OK, [*template-head*](temp.pre#nt:template-head "13.1 Preamble [temp.pre]")*s* matchtemplate<typename T> void S<T>::g() { } // error: no matching declaration for S<T>template<typename T> requires C<T> // ill-formed, no diagnostic required: [*template-head*](temp.pre#nt:template-head "13.1 Preamble [temp.pre]")*s* arevoid S<T>::h() { } // functionally equivalent but not equivalenttemplate<C X> template<D Y>struct S<X>::Inner { }; // OK â *end example*]

â *end note*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L2742)

In a partial specialization,
explicit specialization or
explicit instantiation of a class template,
the [*class-key*](class.pre#nt:class-key "11.1 Preamble [class.pre]") shall agree in kind with the original class template declaration ([[dcl.type.elab]](dcl.type.elab "9.2.9.5 Elaborated type specifiers"))[.](#4.sentence-1)
