Templates

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

C++ language

 

Templates

parameters and arguments

class templates

function templates

class member templates

variable templates(C++14)

template argument deduction

explicit (full) specialization

partial specialization

dependent names

parameter packs(C++11)

sizeof... (C++11)

fold-expressions(C++17)

SFINAE

Constraints and concepts(concepts TS)

 

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)

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 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 > declaration	(1)
export template < parameter-list > declaration	(2)	(until C++11)

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.

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)