Question

#include <iostream>
#include <string>

using namespace std;

class A
{
public:
    A() { i=1; j=2;};
    A (A &obj) { i= obj.i+100; j= obj.j+100;};
     int i;
     int j;
};

class B:public A
{
public:
    B():A() {i=10; j=20; k=30;};

    B(A &obj) {  A::A(obj); k=10000; };//

    int k;
};

int main()
{ 
    A dog;
    B mouse(dog);
    cout<<mouse.i<<endl;
    cout<<mouse.k<<endl;

    return 0;
}

I try to write a copy constructor for the derived class that takes advantage of the copy constructor for the base class. I expect that mouse.i should be 101, but in fact the compiling result is 1. The value for mouse.k is 10000, which is expected. I was wondering what’s wrong with my code.

Answer 1

In this constructor:

B(A &obj) {  A::A(obj); k=10000; };

A::A(obj); does not initialise the base sub-object; instead, it creates a local object also called obj. It’s equivalent to A::A obj;, which is equivalant to A obj;. [UPDATE: or possibly it does something else, or possibly it’s ill-formed – in any event, it’s wrong.]

You want to use an initialiser list:

B(A & obj) : A(obj), k(10000) {}

Also, you almost certainly want the constructor parameters to be A const &, to allow construction from constant objects or temporaries.