Could someone explain the meaning of "using"?

Question

I do not understand below usage of using in C++. What's difference with typedef? Could someone explain it with some example?

template<typename DataType>
class DataWriter
{
using ObjType = std::function<void(DataType)>
// ...
}

Answer 1

There is no difference in your example to a typedef.

Those are identical:

typedef int a;
using a = int;

In general, it is more versatile though, which is the reason it was introduced:

• It can be templated.

  template<class X> using smart = std::unique_ptr<X>;
  

• It can be used to import symbols into the current scope.

  struct Derived : Base {
      using Base::Fun;
  };