# Templates

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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 parametrized 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 both class and function templates, partial specializations are only allowed for class templates.

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(C++20)(optional) declaration | (1) |
`export` `template` `<` parameter-list `>` declaration | (2) | (until C++11)
`template` `<` parameter-list `>` `concept` concept-name `=` constraint-expression `;` | (3) | (since C++20)

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.
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.
concept-name constraint-expression | - | see constraints and concepts (since C++20)

`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)

The template parameter list may be followed by an optional requires-clause that specifies the constraints on the template arguments. | (since C++20)

| This section is incomplete Reason: core syntax, template parameters, and instantiations, take content common between class_template and function_template

## 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 an 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
- (C++20) if the template-id is non-dependent, the associated constraints are satisfied as specified below.

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)

### 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 template
- 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.

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