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Type alias (since C++11)

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Type aliases are similar to typedefs, however, have the advantage of working with templates.

Contents

Syntax

using Template:sparam = Template:sparam;

Explanation

A type alias declaration introduces a name which can be used as a synonym for a previously-declared type. It does not introduce a new type and it cannot change the meaning of an existing type name.

Example

#include <string>
#include <ios>
#include <vector>
#include <type_traits>
 
// 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 fn = example;
 
// template type alias
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 Container>
void fn2(const Container& c)
{
    typename Container::value_type 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 t) { return 1; } 
 
struct S { virtual ~S() {} };
int main() 
{
    Container<int> c;
    fn2(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
}


See also

Template:cpp/language/dcl list typedef