Class template

A class template defines a family of classes.

A class template by itself is not a type, or an object, or any other entity. No code is generated from a source file that contains only template definitions. In order for any code to appear, a template must be instantiated: the template arguments must be provided so that the compiler can generate an actual class (or function, from a function template).

Explicit instantiation


`template` class name `<` argument-list `> ;`	(1)

`extern` `template` class name `<` argument-list `> ;`	(2)	(since C++11)

1) Explicit instantiation definition

2) Explicit instantiation declaration

An explicit instantiation definition forces instantiation of the class, struct, or union they refer to. It may appear in the program anywhere after the template definition, and for a given argument-list, is only allowed to appear once in the program.

An explicit instantiation declaration (an extern template) prevents implicit instantiations: the code that would otherwise cause an implicit instantiation has to use the explicit instantiation definition provided somewhere else in the program.

Implicit instantiation

When code refers to a template in context that requires a completely defined type, or when the completeness of the type affects the code, and this particular type has not been explicitly instantiated, implicit instantiation occurs. For example, when an object of this type is constructed, but not when a pointer to this type is constructed.

This applies to the members of the class template: unless the member is used in the program, it is not instantiated, and does not require a definition.

template<class T> struct Z {
    void f() {}
    void g(); // never defined
}; // template definition
template struct Z<double>; // explicit instantiation of Z<double>
Z<int> a; // implicit instantiation of Z<int>
Z<char>* p; // nothing is instantiated here
p->f(); // implicit instantiation of Z<char> and Z<char>::f() occurs here.
// Z<char>::g() is never needed and never instantiated: it does not have to be defined

Template Non-type arguments

The following limitations apply when instantiating class templates that have non-type template parameters:

For integral and arithmetic types, the template argument provided during instantiation must be a converted constant expression of the template parameter's type.
For pointers to objects, the template arguments have to designate the address of an object with static storage duration and a linkage (either internal or external), or a constant expression that evaluates to the appropriate null pointer value.
For pointers to functions, the valid arguments are pointers to functions with linkage (or constant expressions that evaluate to null pointer values).
For lvalue reference parameters, the argument provided at instantiation cannot be a temporary, an unnamed lvalue, or a named lvalue with no linkage.
For pointers to members, the argument has to be a pointer to member expressed as &Class::Member or a constant expression that evaluates to null pointer value.

In particular, this implies that string literals, addresses of array elements, and addresses of non-static members cannot be used as template arguments to instantiate templates whose corresponding non-type template parameters are pointers to data.

Template type arguments

A template argument for a type template parameter must be a type-id, which may name an incomplete type:

template <class T> class X { }; // class template
 
struct A; // incomplete type
typedef struct {} B; // type alias to an unnamed type
int main() {
   X<A> x1; // OK, 'A' names a type
   X<A*> x2; // OK, 'A*' names a type
   X<B> x3; // OK, 'B' names a type
}

Template template arguments

A template argument for a template template parameter must be an id-expression which names a class template or a template alias.

When the argument is a class template, only the primary template is considered when matching the parameter. The partial specializations, if any, are only considered when a specialization based on this template template parameter happens to be instantiated.

template<class T> class A { // primary template
    int x;
};
template<class T> class A<T*> { // partial specialization
    long x;
};
 
// class template with a template template parameter V
template< template<typename> class V> class C {
    V<int> y; // uses the primary template
    V<int*> z; // uses the partial specialization
};
 
C<A> c; // c.y.x has type int, c.z.x has type long

To match a template template argument A to a template template parameter P, each of the template parameters of A must match corresponding template parameters of P. If P's parameter list includes a parameter pack, zero or more template parameters (or parameter packs) from A's template parameter list are matched by it.

template<class T> struct eval; // primary template 
 
template< template<class, class...> class TT, class T1, class... Rest>
struct eval<TT<T1, Rest...>> {}; // partial specialization of eval
 
template <class T1> struct A;
template <class T1, class T2> struct B;
template <int N> struct C;
template <class T1, int N> struct D;
template <class T1, class T2, int N = 17> struct E;
 
eval<A<int>> eA; // OK: matches partial specialization of eval
eval<B<int, float>> eB; // OK: matches partial specialization of eval
eval<C<17>> eC; // error: C does not match TT in partial specialization because
                // TT's first parameter is a type template parameter,
                // while 17 does not name a type
eval<D<int, 17>> eD; // error: D does not match TT in partial specialization
                     // because TT's second parameter is a type parameter pack,
                     // while 17 does not name a type
eval<E<int, float>> eE; // error: E does not match TT in partial specialization
                        // because E's third (default) parameter is a non-type

Default template arguments

Default template arguments are specified in the parameter lists after the = sign. Defaults can be specified for any kind of template parameter (type, non-type, or template), but not to parameter packs.

If the default is specified for a template parameter of a primary class template, each subsequent template parameter must have a default argument, except the very last one may be a template parameter pack. In a function template, a parameter pack may be followed by more type parameters only if they have defaults or can be deduced from the function arguments.

Default parameters are not allowed

in the out-of-class definition of a member template (they have to be provided in the declaration inside the class body)
in friend class template declarations

On a friend function template declaration, default template arguments are allowed only if the declaration is a definition, and no other declarations of this function appear in this translation unit.

Default template arguments that appear in the declarations and the definition are merged similarly to default function arguments:

template<class T1, class T2 = int> class A;
template<class T1 = int, class T2> class A;
// the above is the same as the following:
template<class T1 = int, class T2 = int> class A;

But the same parameter cannot be given default arguments twice in the same scope

template<class T = int> class X;
template<class T = int> class X { /* ... */ }; // error

The template parameter lists of template template parameters can have their own default arguments, which are only in effect where the template template parameter itself is in scope:

// class template, with a type template parameter with a default
template <class T = float> struct B {};
 
// template template parameter T has a parameter list, which 
// consists of one type template parameter with a default
template <template <class = float> class T> struct A {
    void f();
    void g();
};
// out-of-body member function template definitions
template <template <class TT> class T>
void A<T>::f() {
    T<> t; // error - TT has no default in scope
}
template <template <class TT = char> class T>
void A<T>::g() {
    T<> t; // OK, t is T<char>
}

Examples

Non-type template parameters

Run this code

#include <iostream>
 
// simple non-type template parameter
template<int N>
struct S {
    int a[N];
};
 
template<const char*>
struct S2 {};
 
// complicated non-type example
template <
    char c, // integral type
    int (&ra)[5], // lvalue reference to object (of array type)
    int (*pf)(int), // pointer to function
    int (S<10>::*a)[10] // pointer to member object (of type int[10])
> struct Complicated {
    // calls the function selected at compile time
    // and stores the result in the array selected at compile time
    void foo(char base) {
        ra[4] = pf(c - base);
    }
};
 
//  S2<"fail"> s2; // Error: string literal cannot be used
char okay[] = "okay"; // static object with linkage
// S2< &okay[0] > s2; // Error: array element has no linkage
S2<okay> s2; // works
 
int a[5];
int f(int n) { return n;}
int main()
{
    S<10> s; // s.a is an array of 10 int
    s.a[9] = 4;
 
    Complicated<'2', a, f, &S<10>::a> c;
    c.foo('0');
 
    std::cout << s.a[9] << a[4] << '\n';
}

Output:

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries


type name(optional)	(1)

type name(optional) `=` default	(2)

type `...` name(optional)	(3)	(since C++11)


`typename` name(optional)	(1)

`class` name(optional)	(2)

`typename\|class` name(optional) `=` default	(3)

`typename\|class` `...` name(optional)	(4)	(since C++11)


`template` `<` parameter-list `>` `class` name(optional)	(1)

`template` `<` parameter-list `>` `class` name(optional) `=` default	(2)

`template` `<` parameter-list `>` `class` `...` name(optional)	(3)	(since C++11)

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Syntax

Explanation

Non-type template parameter

Type template parameter

Template template parameter

Class template instantiation