Question

I have a huge C++ code base I’m working with.

I want to introduce a new subclass, and use it in a variety of different contexts, however my Java trained mind is having trouble with this in C++.

I wish to minimize code changes. So, my main class of interest (BAR below) has a member variable of class FOO. I wish to make a subclass of FOO called FOOSUBCLASS. In java this is trivial. Objects are stored by reference by default. This code does not do that (as seen below) can I massage this code, without changing interfaces (and without introducing references) and still make my application work?

class FOO {
};

class FOOSUBCLASS : FOO {
public:
    FOOSUBCLASS(const int id) {_id = id;}
private:
    int _id;
};

class BAR {
public:
    BAR(const FOO foo) { _foo = foo;}
private:
    FOO _foo;
};

Below, is my main:

FOOSUBCLASS fsc(1);
BAR b(fsc);

But this doesn’t compile in VS2005. It says:

‘type cast’ : conversion from ‘FOOSUBCLASS *’ to ‘const FOO &’ exists, but is inaccessible

Get same compiler error if I add the obvious other constructor to BAR

BAR(const FOOSUBCLASS foo) { _foo = foo;}

My understanding of C++ is that it will copy the data from FOOSUBCLASS into an object of class FOO. (using the assignment operator, or a class overridden version of it) But in my case, I have additional member variables in FOOSUBCLASS (and some overridden member functions) so I just don’t want it to do that. I want my member variable _foo to truly be of type FOOSUBCLASS in some contexts, and FOO in others. Is this even possible?

Thanks for your thoughts.

Answer 1

You can’t do it the way you’ve written it now, sorry. This is called “slicing”, and more info about it can be found here: What is object slicing?

If you want your _foo member to be polymorphic, you’d need to use a pointer or a reference. Be aware of lifetime management when you do things like this of course.