Thread: assignment constructor overloading

I guess it works now, when I compile it again... my only problem now is this:

I have a method called print and printit. A and B each has a method called print and printit is in main

Code:

void printit( A& a )
{
 a.print( );
}

Here's part of main's code

Code:

B b1, b2(2);
b1 = b2;
A a; a = b1;
b1.print( ); a.print( );

it would print:
2
0

my confusion is why when doing a.print() a is 0, shouldn't it be 2 as we already assign a to b1? and b1 has a value2? how can I make it so that it has a value 0?

Why would the base class need to know anything about the derived class?
There must be a better way to design it.

Originally Posted by tabstop

Did you slice off the value 2 when you copied b1 to a?

Can you give me an example of that? And how to fix this?

Originally Posted by -EquinoX-

Can you give me an example of that?

Consider:

Code:

#include <ostream>

class A
{
public:
    virtual ~A() {}

    virtual void print(std::ostream& out) const
    {
        out << 0;
    }
};

class B : public A
{
public:
    explicit B(int x) : x(x) {}

    virtual void print(std::ostream& out) const
    {
        out << x;
    }
private:
    int x;
};

#include <iostream>

int main()
{
    A a;
    B b(1);
    a = b;
    a.print(std::cout);
    std::cout << std::endl;
}

Since a is an A, it cannot possibly have a member variable named x since no object of actual type A has such a member variable. a.print(std::cout) must then print 0, since it results in A::print(std::ostream) being invoked. If it could result in B::print(std::ostream) being invoked, then what should x be?

Originally Posted by -EquinoX-

And how to fix this?

So, the problem is that the parts of b specific to B are "sliced off" when b is assigned to a. If your intention is to use polymorphism, then you should do something like:

Code:

int main()
{
    A* a;
    B b(1);
    a = &b;
    a->print(std::cout);
    std::cout << std::endl;
}

since such polymorphism in C++ works through pointers and references.

Originally Posted by -EquinoX-

is there a way to fix this without changing the main

Probably not, at least not without hacks that will likely over-complicate your code.

Originally Posted by -EquinoX-

when we assign a to b a doesn't have the value that b has, is this the problem?

Yes. More generally, the part of B that is specific to B will be sliced off.

in this code, how can I make so that when I call printit(b2) it prints a 2 instead of 0?

Code:

void printit( A& a )
{
 a.print( );
}

Code:

B b1, b2(2);
b1 = b2;
A a; a = b1;
b1.print( ); a.print( );

when I do printit(b2) it uses the print method that A has and so it prints a value of A which is 0... which is not right

If this is with respect to my definitions of A and B in post #7, then it is as simple as:

Code:

printit(b2);