Derived to base class copy

[venomxxl]

Hello
I'm not new to C++ programming, but I have a question for you guys and gals :) So the code below compiles fine (due to that ugly workaround in main func), but that makes d_copy.field2 unitialized (0xCC bytes in MSVC). How would you change the code to make d equal to d_copy? You can't change the type of the global variable and you must use it to copy d. However, you can use pointers, references and whatever pleases you. I guess set and get functions are optional, but I'd love to see them too. Is that possible in C++? If not, please let me know! ;)

class CBase
{
public:
    CBase() : field1(0) {}
    ~CBase() {}

    int field1;
};

class CDerived : 
    public CBase
{
public:
    CDerived() : field2(0) {}
    ~CDerived() {}

    int field2;
};

CBase b;

void func_set(CBase *pb)
{
    b = *pb;
}

CBase func_get()
{
    return b;
}

int main()
{
    CDerived d;
    CDerived d_copy;

    d.field1 = 321;
    d.field2 = 123;

    func_set(&d);
    d_copy = *(CDerived*)(&func_get());

    return 0;
}

If the question is unclear to you, please let me know.

[deceptikon]

If the question is unclear to you, please let me know.

The point of this exercise is unclear to me. Is it just random homework from a teacher who thinks trivia will somehow make you a better programmer? Or do you have an actual real world need?

[np complete]

Change line 24 to b.field1 = (*pb).field1; . It then worked for me. And what is the motive of this exercise ?

[venomxxl]

I need it for my own project. This isnt a homework. Basically I want to have a class that stores the base class object and use it with derived objects in the manner Ive shown in the example. Ive already solved the problem with templates but I was wondering if you could do it that way. Ill provide more info later if necessary.

[NathanOliver]

You probolly can get away with changing line 24 to

&b = pb;

[vijayan121]

CBase b;

void func_set(CBase *pb) 
// pb may point to an object which has the dynamic type of some derived class of CBase
{
    b = *pb; // the object *pb may be sliced here
}

See http://stackoverflow.com/questions/274626/what-is-the-slicing-problem-in-c

You probolly can get away with changing line 24 to
&b = pb;

&b is not an lvalue.

[venomxxl]

That explains why d_copy.field2 is uninitialized. I was already aware of that, but thanks for pointing that out. So a template class is the only reasonable way to solve this problem? If it is so, then I guess this thread can be closed. I'll wait a little longer then I'll mark this question as solved.