Strange behaviour of copy constructor

Question

I'm new to c++ programming and like others i find the copy constructor concept a bit stange.I went through a site which said

The copy constructor is a special kind of constructor which creates a new object which is a copy of an existing one, and does it efficiently.

I wrote a code to create an object which is a copy of an other object and found the results to be strange the code is as below .

 #include <iostream>

 using namespace std;

 class box
 {
   public:

   double getwidth();
   box(double );
   ~box();
    box(box &);
   private:
      double width;
 };

box::box(double w)
{
   cout<<"\n I'm inside the constructor ";
   width=w;
}

box::box(box & a)
{
  cout<<"\n Copy constructructor got activated ";
}
box::~box()
{
  cout<<"\n I'm inside the desstructor ";

}

double box::getwidth()
{
   return width;
}


int main()
{
  box box1(10);
  box box2(box1);
  cout<<"\n calling getwidth from first object  : " <<box1.getwidth();
  cout<<"\n calling the getwidth from second object   : " <<box2.getwidth(); 
}

When i called box2.getwidth() as per the code below i got a junk value . I expected the width to be initialized to 10 as the box2 is copy of box1 as per my understanding .Please clarify

Answer 1

Your expectation was that all members are copied automagically, but they're not (not if you provide your own implementation). You'll need to add the logic yourself:

box::box(const box & a)
{
  width = a.width;
  cout<<"\n Copy constructructor got activated ";
}

Your version tells the compiler - "whenver you make a copy, print out that thing", which it does. You never instruct it to copy any of the members.

Just FYI, if you provide no implementation, the compiler will generate a copy constructor for you, which does a shallow, memberwise copy.

Answer 2

Write your copy ctor like this. Your copy constructor code doesn't copy the object contents.

box::box(box & a):width(a.width)
{
  cout<<"\n Copy constructructor got activated ";
}
int main()
{
box a(10);

box b = a;//copy constructor is called
return 0;
}