Operator Overloading Problem

[m4rtin]

I have inherited a project from VC++ 6.0 and am trying to bring it into .NET. I am having a problem with the compiler returning the following error in an operator overload:

error C2240: 'static_cast' cannot convert from 'const CObject*' to 'CSomeObject2*'

Where CObject is actually the base class CObject and CSomeObject2 is another class. I was hoping someone might be able to understand how the following code should work as I cannot see how it ever worked, but this is probably due to my misunderstanding.

void CSomeObject2::operator =(const CSomeObject1& baseSrc) // Line 1
{ // Line 2
CSomeObject2& src = *STATIC_DOWNCAST(CSomeObject2, &baseSrc); // Line 3
CSomeObject1::operator =(src); // Line 4
property = src.property; // Line 5
}

Also, CSomeObject2 is derived from CSomeObject1 and CSomeObject1 is derived from CObject. My problem lies on Line 3. I basically agree with the compiler; however, I don't see how this code ever worked. I know it worked in VC++ 6.0, but how? Could someone please explain. Thanks.

[Athar]

Are you sure you "translated" the code correctly? The code piece makes little sense.
Also, what the hell is STATIC_DOWNCAST supposed to be? Not only does it seem to perform an upcast, but the error message does not fit the code. In any case, dynamic_cast is the correct operator to use here.

Besides, this function will only work correctly if the passed object is actually of type CSomeObject2.
So the parameter should have been a reference to CSomeObject2 in the first place.

[m4rtin]

STATIC_DOWNCAST performs a similar function to DYNAMIC_DOWNCAST; however, the STATIC version will result in an ASSERT when debugging if the result is invalid. Also, the code was translated correctly as I double checked before I submitted it, but thank you for ensuring that was the case. Any more explanations would be greatly appreciated.

[Athar]

Neither STATIC_DOWNCAST nor DYNAMIC_DOWNCAST are part of C++, so you'll have to post the appropriate source code.

Besides, it's pointless trying to figure out why VC++6 managed to compile it - that compiler was notorious for not conforming to the C++ standard.

[m4rtin]

STATIC_DOWNCAST and DYNAMIC_DOWNCAST are simply MFC macros and achieve the same basic functionality as static_cast and dynamic_cast.

[kbw]

STATIC_DOWNCAST and DYNAMIC_DOWNCAST exist in MFC because the compiler didn't support it when MFC needed it, so they rolled their own using a feature built into CObject (IsKindOf).

The world has changed. Even Microsoft have implemented static_cast and dynamic_cast. It's a good idea to use those and ditch the macros unless for some specific reason you don't want to enable RTTI.

[m4rtin]

I think I may not have stated my question propertly, but the problem that I was having was the const CSomeObject1 being passed by reference and then it's downcasting in Line 3. Why would you ever pass a const object and then try and modify it in the function, I don't see how this ever worked. Maybe I'm looking to explain the unexplainable, lol. Thanks for the replies so far.

[kbw]

It worked becase C style casts do whatever it takes; that's the problem with them.

The new C++ style casts don't, thus saving us from ourselves.