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Class template

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A class template defines a family of classes.

Contents

Syntax

template < parameter-list > declaration (1)
export template < parameter-list > declaration (2) (until C++11)

Explanation

declaration defines or declares a class (including struct and union), a member class or member enumeration type, a function or member function, a static data member of a class template, or a alias template. It may also define a template specialization. This page focuses on class templates.

parameter-list is a non-empty comma-separated list of the template parameters, each of which is either non-type parameter, a type parameter, a template parameter, or a parameter pack of any of those. This page focuses on the parameters that are not parameter packs.

(until C++11)export was an optional modifier which declared the template as exported (when used with a class template, it declared all of its members exported as well). Files that instantiated exported templates did not need to include their definitions: the declaration was sufficient. Implementations of export were rare and disagreed with each other on details.

Non-type template parameter

type name(optional) (1)
type name(optional) = default (2)
type ... name(optional) (3) (since C++11)
1) A non-type template parameter with an optional name
2) A non-type template parameter with an optional name and a default value
3) A non-type template parameter pack with an optional name

type is one of the following types (optionally cv-qualified, the qualifiers are ignored)

  • integral type
  • enumeration
  • pointer to object or to function
  • lvalue reference to object or to function
  • pointer to member object or to member function
  • std::nullptr_t (since C++11)

Array and function types may be written in a template declaration, but they are automatically replaced by pointer to data and pointer to function as appropriate.

When the name of a non-type template parameter is used in an expression within the body of the class template, it is an unmodifiable prvalue unless its type was an lvalue reference type.

Type template parameter

typename name(optional) (1)
class name(optional) (2)
typename|class name(optional) = default (3)
typename|class ... name(optional) (4) (since C++11)
1) A type template parameter with an optional name
2) Exactly the same as 1)
3) A type template parameter with an optional name and a default
5) A type template parameter pack with an optional name

In the body of the template declaration, the name of a type parameter is a typedef-name which aliases the type supplied when the template is instantiated.

There is no difference between the keywords class and typename in a type template parameter declaration.

Template template parameter

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)
1) A template template parameter with an optional name
2) A template template parameter with an optional name and a default
3) A template template parameter pack with an optional name

Unlike type template parameter declaration, template template parameter declaration can only use the keyword class and not typename.

In the body of the template declaration, the name of this parameter is a template-name (and needs arguments to be instantiated)

template<class T> class myarray {};
 
// two type template parameters and one template template parameter:
template<class K, class V, template<typename> class C = myarray>
class Map {
  C<K> key;
  C<V> value;
};

Class template instantiation

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 entire 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 (typically, in another file: this can be used to reduce compilation times) (since C++11)

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

#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:

42

See also