Base Copy constructor not called

Question

class Base
{
      public:
      int i;

      Base()
      {
          cout<<"Base Constructor"<<endl;
      }

      Base (Base& b)
      {
          cout<<"Base Copy Constructor"<<endl; 
          i = b.i; 
      }


      ~Base()
      {
          cout<<"Base Destructor"<<endl;
      }

      void val()
      {
             cout<<"i: "<< i<<endl;
      }      
};

class Derived: public Base
{
      public:
      int i;

      Derived()
      {
          Base::i = 5;     
          cout<<"Derived Constructor"<<endl;
      }

      /*Derived (Derived& d)
      {
          cout<<"Derived copy Constructor"<<endl;
          i = d.i; 
      }*/

      ~Derived()
      {
          cout<<"Derived Destructor"<<endl;
      }      

      void val()
      {
             cout<<"i: "<< i<<endl;
             Base::val();
      }
};

If i do Derived d1; Derived d2 = d1; The copy constructor of base is called and default copy constructor of derived is called.

But if i remove the comments from derived's copy constructor the base copy constructor is not called. Is there any specific reason for this? Thanks in advance.

Answer 1

I think you have to explicitly call the base copy constructor:

  Derived (Derived& d) : Base(d)
  {
      cout<<"Derived copy Constructor"<<endl;
      i = d.i; 
  }

Answer 2

If you want to read actual rule you should refer to C++ Standard 12.8/8:

The implicitly-defined copy constructor for class X performs a memberwise copy of its subobjects. The order of copying is the same as the order of initialization of bases and members in a user-defined construc- tor (see 12.6.2). Each subobject is copied in the manner appropriate to its type:

• if the subobject is of class type, the copy constructor for the class is used;
• if the subobject is an array, each element is copied, in the manner appropriate to the element type;
• if the subobject is of scalar type, the built-in assignment operator is used.

When you define copy constructor explicitly you should call copy c-tor of base class explicitly.