在派生类构造函数中复制继承的成员是否有效？

Question

In the below code I have defined an explicit copy constructor in the derived class. I have also written my own copy constructor in the base class.

Primer says that the derived copy constructor must call the base one explicitly and that inherited members should be copied by the base class copy constructor only, but I copied the inherited members in the derived class copy constructor and it is working fine. How is this possible?

Also how do I explicitly call the base class copy constructor inside the derived class copy constructor? Primer says base(object), but I am confused how the syntax differentiates between normal constructor call and copy constructor call.

Thanks in advance.

#include<stdafx.h>
#include<iostream>

using namespace std;

class A
{
public:
  int a;
  A()
  {
    a = 7;
  }

  A(int m): a(m)
  {
  }
};

class B : public A
{
public:
  int b;
  B()
  {
    b = 9;
  }

  B(int m, int n): A(m), b(n)
  {
  }

  B(B& x)
  {
    a = x.a;
    b = x.b;
  }

  void show()
  {
    cout << a << "\t" << b << endl;
  }
};

int main()
{
  B x;
  x = B(50, 100);
  B y(x);
  y.show();
  return 0;
}

Answer 1

copy constructor is when you have another Object passed to your constructor:

class A() {
    private:
        int a;
    public:
        //this is an empty constructor (or default constructor)
        A() : a(0) {};
        //this is a constructor with parameters
        A(const int& anotherInt) : a(anotherInt) {};
        //this is a copy constructor
        A(const A& anotherObj) : a(anotherObj.a) {}; 
}

for a derived class

class B : public A {
    private:
         int b;
    public:
         //this is the default constructor
         B() : A(), b() {};
         //equivalent to this one
         B() {};
         //this is a constructor with parameters
         // note that A is initialized through the class A.
         B(const int& pa, const int& pb) : A(pa), b(pb) {}
         //for the copy constructor
         B(const B& ob) : A(ob), b(ob.b) {}        
}

Answer 2

How somebody else wrote here:

How to call base class copy constructor from a derived class copy constructor?

I would write the copy constructor like this instead how it was written by macmac:

B(const B& x) : A(x) , b(x.b)
{
}

To call the copy constructor of the base A you simply call it passing the derived B object A(B), is not neccessary to specify Ba

EDIT: macmac edited his answere in the correct way, now his answere is better then mine.