Question

I have these classes:

first:

 class C 
 {
     public:
     C(const C& c):_s(c._s){}
     c():_s(""){}

     string _s;
 }

second:

class C2: public C
{
    public:
    C2(const C2 & c2):_i(c2.i){}
    C2():_i(0){}

    int _i;
}

main:

int main()
{
    C2 c2;
    C2._s="hello";
    c2._i=42;
    C2 c3(c2);
    cout<<c3._s<<" "<<c3._i<<endl;
}

and the output is 42. My question is, why is the output 42? A base constructor is always called before the derived constructors, so this line:

C2 c3(c2);

should call C‘s copy constructor and should copy “hello”, meaning the output should be hello. What am I missing out here?

Answer 1

The reason is

C2(const C2 & c2):_i(c2.i){}

doesn’t initialize _s to the values stored in c2 because the default (parameterless) constructor of base class is called unless specified otherwise and so _s is also initialized with its default constructor, not copy constructor.

Specifically in your case C2::(const C2&) invokes C::C() and that invokes string::string().

You have to explicitly call the right base constructor:

C2(const C2 & c2) :
  C( c2 ),
  _i(c2.i)
{
}