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Showing content from https://en.cppreference.com/w/cpp/language/../memory/shared_ptr/../../language/type_alias.html below:

Type alias, alias template (since C++11)

Type alias is a name that refers to a previously defined type (similar to typedef).

Alias template is a name that refers to a family of types.

Alias declarations are declarations with the following syntax:

using identifier attr (optional) = type-id ;

using identifier attr (optional) = type-id ;

A type alias declaration introduces a name which can be used as a synonym for the type denoted by

. 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.

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

.

template<class T>
struct Alloc {};
 
template<class T>
using Vec = 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 template-id, subsequent substitutions apply to that template-id:

template<typename...>
using void_t = void;
 
template<typename T>
void_t<typename T::foo> f();
 
f<int>(); // error, int does not have a nested type foo

The type produced when specializing an alias template is not allowed to directly or indirectly make use of its own type:

template<class T>
struct A;
 
template<class T>
using B = typename A<T>::U; // type-id is A<T>::U
 
template<class T>
struct A { typedef B<T> U; };
 
B<short> b; // error: B<short> uses its own type via A<short>::U

Alias templates are never deduced by template argument deduction 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 lambda expression appearing in an alias template declaration is different between instantiations of that template, even when the lambda expression is not dependent.

template<class T>
using A = decltype([] {}); // A<int> and A<char> refer to different closure types

using

#include <iostream>
#include <string>
#include <type_traits>
#include <typeinfo>
 
// type alias, identical to
// typedef std::ios_base::fmtflags flags;
using flags = std::ios_base::fmtflags;
// the name 'flags' now denotes a type:
flags fl = std::ios_base::dec;
 
// type alias, identical to
// typedef void (*func)(int, int);
using func = void (*) (int, int);
 
// the name 'func' now denotes a pointer to function:
void example(int, int) {}
func f = example;
 
// alias template
template<class T>
using ptr = T*;
// the name 'ptr<T>' is now an alias for pointer to T
ptr<int> x;
 
// type alias used to hide a template parameter
template<class CharT>
using mystring = std::basic_string<CharT, std::char_traits<CharT>>;
 
mystring<char> str;
 
// type alias can introduce a member typedef name
template<typename T>
struct Container { using value_type = T; };
 
// which can be used in generic programming
template<typename ContainerT>
void info(const ContainerT& c)
{
    typename ContainerT::value_type T;
    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_if
template<typename T>
using Invoke = typename T::type;
 
template<typename Condition>
using EnableIf = Invoke<std::enable_if<Condition::value>>;
 
template<typename T, typename = EnableIf<std::is_polymorphic<T>>>
int fpoly_only(T) { return 1; }
 
struct S { virtual ~S() {} };
 
int main()
{
    Container<int> c;
    info(c); // Container::value_type will be int in this function
//  fpoly_only(c); // error: enable_if prohibits this
    S s;
    fpoly_only(s); // okay: enable_if allows this
}

Possible output:

ContainerT is `struct Container<int>`
value_type is `int`

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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