Question

I’m setting a up an interface for various components of a framework in a personal project, and i’ve suddenly thought of something that i figured might be useful with an interface. My question is whether this is possible or not:

class a
{
public:
    virtual class test = 0;

};

class b : public a
{
public:
    class test
    {
        public:
           int imember;
    };
};

class c : public a
{
public:
    class test
    {
    public:
           char cmember;  // just a different version of the class. within this class
    };
};

sort of declaring a virtual class or pure virtual class, that is required to be defined within the derived object, so that you might be able to do something like this:

int main()
{
    a * meh = new b();
    a * teh = new c();

    /* these would be two different objects, but have the same name, and still be able  
 to be referred to by an interface pointer in the same way.*/
    meh::test object1;    
    teh::test object2;

    delete meh;
    delete teh;

    return 0;
}

msvc++ throws me a bunch of syntax errors, so is there a way to do this, and i’m just not writing it right?

Answer 1

No, it isn’t valid. In any case, C++ has no concept of virtual classes as such. You can probably achieve what you want by holding a pointer to a certain class with only pure virtual methods (although that isn’t a requirement):

class ITest { /* full of pure virtual methods... maybe. */};

class a
{
public:
    virtual ITest* someFunctionName()=0 ;
private:
    ITest* test_;
};

Then you can decide to inherit from a, giving each implementation concrete implementations of ITest, or some other approach, such as deciding which implementation to use based on some constructor parameter, for example.