From cppreference.com
Type alias is a name that refers to a previously defined type (similar to
).
Alias template is a name that refers to a family of types.
Syntax
Alias declarations are
with the following syntax:
usingidentifierattr(optional)=type-id;(1) template<template-parameter-list>usingidentifierattr(optional)=type-id;
(2) template<template-parameter-list>requiresconstraintusingidentifierattr(optional)=type-id;
(3) (since C++20)attr- optional sequence of any number of
identifier- the name that is introduced by this declaration, which becomes either a type name (1) or a template name (2)template-parameter-list-
, as in
constraint- a
which restricts the template parameters accepted by this alias template type-id- abstract declarator or any other valid type-id (which may introduce a new type, as noted in
). The type-id cannot directly or indirectly refer to identifier. Note that the
of the identifier is at the semicolon following type-id. Explanation
1) A type alias declaration introduces a name which can be used as a synonym for the type denoted by type-id. It does not introduce a new type and it cannot change the meaning of an existing type name. There is no difference between a type alias declaration and
declaration. This declaration may appear in block scope, class scope, or namespace scope.
2) An alias template is a template which, when specialized, is equivalent to the result of substituting the template arguments of the alias template for the template parameters in the type-id.
template<classT>structAlloc{};template<classT>usingVec=vector<T,Alloc<T>>;// type-id is vector<T, Alloc<T>>Vec<int>v;// Vec<int> is the same as vector<int, Alloc<int>>When the result of specializing an alias template is a dependent
, subsequent substitutions apply to that template-id:
template<typename...>usingvoid_t=void;template<typenameT>void_t<typenameT::foo>f();f<int>();// error, int does not have a nested type fooThe type produced when specializing an alias template is not allowed to directly or indirectly make use of its own type:
template<classT>structA;template<classT>usingB=typenameA<T>::U;// type-id is A<T>::Utemplate<classT>structA{typedefB<T>U;};B<short>b;// error: B<short> uses its own type via A<short>::UAlias templates are never deduced by
when deducing a template template parameter.
It is not possible to
or
an alias template.
Like any template declaration, an alias template can only be declared at class scope or namespace scope.
The type of a
appearing in an alias template declaration is different between instantiations of that template, even when the lambda expression is not dependent.
template<classT>usingA=decltype([]{});// A<int> and A<char> refer to different closure types(since C++20)Notes
Feature-test macroValueStdFeature
(C++11)Alias templates Keywords
Example
Run this code
#include<iostream>#include<string>#include<type_traits>#include<typeinfo>// type alias, identical to// typedef std::ios_base::fmtflags flags;usingflags=std::ios_base::fmtflags;// the name 'flags' now denotes a type:flagsfl=std::ios_base::dec;// type alias, identical to// typedef void (*func)(int, int);usingfunc=void(*)(int,int);// the name 'func' now denotes a pointer to function:voidexample(int,int){}funcf=example;// alias templatetemplate<classT>usingptr=T*;// the name 'ptr<T>' is now an alias for pointer to Tptr<int>x;// type alias used to hide a template parametertemplate<classCharT>usingmystring=std::basic_string<CharT,std::char_traits<CharT>>;mystring<char>str;// type alias can introduce a member typedef nametemplate<typenameT>structContainer{usingvalue_type=T;};// which can be used in generic programmingtemplate<typenameContainerT>voidinfo(constContainerT&c){typenameContainerT::value_typeT;std::cout<<"ContainerT is `"<<typeid(decltype(c)).name()<<"`\n""value_type is `"<<typeid(T).name()<<"`\n";}// type alias used to simplify the syntax of std::enable_iftemplate<typenameT>usingInvoke=typenameT::type;template<typenameCondition>usingEnableIf=Invoke<std::enable_if<Condition::value>>;template<typenameT,typename=EnableIf<std::is_polymorphic<T>>>intfpoly_only(T){return1;}structS{virtual~S(){}};intmain(){Container<int>c;info(c);// Container::value_type will be int in this function// fpoly_only(c); // error: enable_if prohibits thisSs;fpoly_only(s);// okay: enable_if allows this}Possible output:
ContainerT is `struct Container<int>` value_type is `int` Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++11 whether unused arguments in an alias specialization
participate in substitution was not specified substitution
is performed See also