c++ - 基类构造函数的 using 声明

Question

乍一看，using 用于导入特定函数(例如using std::cout 到作用域)。但是这个using实际上是将基类的所有构造函数导入到派生类中。这个 using 声明的幕后究竟是什么？

template< typename T >
class Vec : public std::vector< T >
{
public:
    using std::vector<T>::vector;  // ?

    //...
};

Answer 1

As it's public inherited, supposedly all the base class constructors should have been available already (ie why need using)?

No，默认不继承基类的构造函数。可以在以下讨论中找到详细解释:

• Why are constructors notinherited?
• Why aren't C++ constructorsinherited?

---

What's actually behind the scene of this using declaration?

来自 cppreference.com ，使用可以

1. Using-declarations can be used to introduce namespace members into other namespaces and block scopes, or to introduce base class members into derived class definitions.
2. [...] (c++20 specific...)

同时 inheritance :

If the using-declaration refers to a constructor of a direct base of the class being defined (e.g. using Base::Base;), all constructors of that base (ignoring member access) are made visible to overload resolution when initializing the derived class.