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Special member functions

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 does not 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).


[edit] 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(since C++11).
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)

[edit] Template identifiers

A template identifier has one of the following syntaxes:

template-name <template-argument-list (optional)> (1)
operatorop <template-argument-list (optional)> (2)
operator "" identifier <template-argument-list (optional)> (3) (since C++11)
operator user-defined-string-literal <template-argument-list (optional)> (4) (since C++11)
1) A simple template identifier.
2) An operator function template identifier.
3,4) A literal operator function template identifier.
template-name - an identifier that names a template
op - an overloadable operator
identifier - an identifier
user-defined-string-literal - "" followed by an identifier

A simple template identifier that names a class template specialization names a class.

A template identifier that names an alias template specialization names a type.

A template identifier that names a function template specialization names a function.

If all following conditions are satisfied, a template identifier is valid :

  • There are at most as many arguments as there are parameters or a parameter is a template parameter pack(since C++11).
  • There is an argument for each non-deducible non-pack(since C++11) 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.
  • If the template identifier 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)

If all following conditions are satisfied, two template identifiers are same :

  • Their template-name s or operators refer to the same template.
  • Their corresponding type template arguments are the same type.
  • The template parameter values determined by their corresponding non-type template arguments are template-argument-equivalent.
  • Their corresponding template template arguments refer to the same template.

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

[edit] 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)

[edit] 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
identifier is valid were not provided
CWG 2682 C++98
the definitions of templated function/template class
(C++98)/templated variable (C++14) were missing
P2308R1 C++98 two template identifiers were different if their
corresponding non-type template arguments
are not template-argument-equivalent
they are different if their corresponding
non-type template parameter values
are not template-argument-equivalent

[edit] See also

C documentation for Generic selection