Templates

 

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Templates

Parameters and arguments

Class templates

Function templates

Class member templates

Variable templates (C++14)

Template argument deduction

Class template argument deduction (C++17)

Explicit (full) specialization

Partial specialization

Dependent names

Parameter packs (C++11)

sizeof... (C++11)

Fold-expressions (C++17)

SFINAE

Constraints and concepts (C++20)

Requires expression (C++20)

 

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

• a family of classes (class template), which may be nested classes
• a family of functions (function template), which may be member functions

an alias to a family of types (alias template)

(since C++11)

a family of variables (variable template)

(since C++14)

a concept (constraints and concepts)

(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 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).

Syntax

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.
concept-name constraint-expression	-	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-id

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-ids are same if

• their template-names 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-ids 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) within a template definition. All of the following are templated entities:

• a class/function/variable (since C++14) template

a concept

(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

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.

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