A template is a C++ entity that defines one of the following:

(since C++11)
(since C++14)
(since C++20)

Templates are parameterized by one or more template parameters, of three kinds: type template parameters, non-type template parameters, and template template parameters.

When template arguments are provided, or, for function and class (since C++17) templates only, deduced, they are substituted for the template parameters to obtain a specialization of the template, that is, a specific type or a specific function lvalue. Specializations may also be provided explicitly: full specializations are allowed for class, variable (since C++14) and function templates, partial specializations are only allowed for class templates and variable templates (since C++14).

When a class template specialization is referenced in context that requires a complete object type, or when a function template specialization is referenced in context that requires a function definition to exist, the template is instantiated (the code for it is actually compiled), unless the template was already explicitly specialized or explicitly instantiated. Instantiation of a class template doesn't instantiate any of its member functions unless they are also used. At link time, identical instantiations generated by different translation units are merged.

The definition of a class template must be visible at the point of implicit instantiation, which is why template libraries typically provide all template definitions in the headers (e.g. most boost libraries are header-only).


template <parameter-list > requires-clause (optional) declaration (1)
export template <parameter-list > declaration (2) (until C++11)
template <parameter-list > concept concept-name = constraint-expression ; (3) (since C++20)
parameter-list - 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.
requires-clause - (since C++20) a requires-clause that specifies the constraints on the template arguments.
declaration - declaration of a class (including struct and union), a member class or member enumeration type, a function or member function, a static data member at namespace scope, a variable or static data member at class scope, (since C++14) or an alias template (since C++11) It may also define a template specialization.
- see constraints and concepts

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.

(until C++11)


template-name <parameter-list >
template-name - either an identifier that names a template (in which case this is called "simple-template-id ") or a name of an overloaded operator template or user-defined literal template.

A simple-template-id that names a class template specialization names a class.

A template-id that names an alias template specialization names a type.

A template-id that names a function template specialization names a function.

A template-id is only valid if.

  • there are at most as many arguments as there are parameters or a parameter is a template parameter pack,
  • there is an argument for each non-deducible non-pack parameter that does not have a default template-argument,
  • each template-argument matches the corresponding template-parameter,
  • substitution of each template argument into the following template parameters (if any) succeeds, and
  • if the template-id is non-dependent, the associated constraints are satisfied as specified below.
(since C++20)

An invalid simple-template-id is a compile-time error, unless it names a function template specialization (in which case SFINAE may apply).

template<class T, T::type n = 0>
class X;
struct S
    using type = int;
using T1 = X<S, int, int>; // error: too many arguments
using T2 = X<>;            // error: no default argument for first template parameter
using T3 = X<1>;           // error: value 1 does not match type-parameter
using T4 = X<int>;         // error: substitution failure for second template parameter
using T5 = X<S>;           // OK

When the template-name of a simple-template-id names a constrained non-function template or a constrained template template-parameter, but not a member template that is a member of an unknown specialization, and all template-arguments in the simple-template-id are non-dependent, the associated constraints of the constrained template must be satisfied:

template<typename T>
concept C1 = sizeof(T) != sizeof(int);
template<C1 T>
struct S1 {};
template<C1 T>
using Ptr = T*;
S1<int>* p;                      // error: constraints not satisfied
Ptr<int> p;                      // error: constraints not satisfied
template<typename T>
struct S2 { Ptr<int> x; };       // error, no diagnostic required
template<typename T>
struct S3 { Ptr<T> x; };         // OK, satisfaction is not required
S3<int> x;                       // error: constraints not satisfied
template<template<C1 T> class X>
struct S4
    X<int> x;                    // error, no diagnostic required
template<typename T>
concept C2 = sizeof(T) == 1;
template<C2 T> struct S {};
template struct S<char[2]>;      // error: constraints not satisfied
template<> struct S<char[2]> {}; // error: constraints not satisfied
(since C++20)

Two template-id s are same if.

  • their template-name s refer to the same template, and
  • their corresponding type template arguments are the same type, and
  • their corresponding non-type template arguments are template-argument-equivalent after conversion to the type of the template parameter, and
  • their corresponding template template arguments refer to the same template.

Two template-id s that are the same refer to the same variable, (since C++14) class, or function.

Templated entity

A templated entity (or, in some sources, "temploid") is any entity that is defined (or, for a lambda expression, created) (since C++11) within a template definition. All of the following are templated entities:

  • a class/function/variable (since C++14) template
(since C++20)
  • a member of a templated entity (such as a non-template member function of a class template)
  • an enumerator of an enumeration that is a templated entity
  • any entity defined or created within a templated entity: a local class, a local variable, a friend function, etc
  • the closure type of a lambda expression that appears in the declaration of a templated entity
(since C++11)

For example, in.

template<typename T>
struct A
    void f() {}

the function A::f is not a function template, but is still considered to be templated.

A templated function is a function template or a function that is templated.

A templated class is a class template or a class that is templated.

A templated variable is a variable template or a variable that is templated.

(since C++14)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 2293 C++98 the rules of determining whether a template-id is valid were not provided provided
CWG 2682 C++98
the definitions of templated function/templated class (C++98)/
templated variable (C++14) were missing

See also

C documentation for Generic selection

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