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