Creating an interface for an abstract class template in C++

Question

I have the code as below. I have a abstract template class Foo and two subclasses (Foo1 and Foo2) which derive from instantiations of the template. I wish to use pointers in

Answer 1

Since functionB's argument type must be known in advance, you have only one choice: Make it a type which can hold every possible argument. This is sometimes called a "top type" and the boost libraries have the any type which gets quite close to what a top type would do. Here is what could work:

#include 
#include 
using namespace boost;

class IFoo {
    public:
    virtual void functionA()=0;
    virtual void functionB(any arg)=0; //<-can hold almost everything
};

template
class Foo : public IFoo{
    public:
        void functionA(){  };
        void real_functionB(T arg)
        {
         std::cout << arg << std::endl;
        };
        // call the real functionB with the actual value in arg
        // if there is no T in arg, an exception is thrown!

        virtual void functionB(any arg)
        {
            real_functionB(any_cast(arg));
        }
};

int main()
{
    Foo f_int;
    IFoo &if_int=f_int;

    if_int.functionB(10);

    Foo f_double;
    IFoo &if_double=f_double;
if_int.functionB(10.0);

}

Unfortunately, any_cast does not know about the usual conversions. For example any_cast(any(123)) throws an exception, because it does not even try to convert the integer 123 to a double. If does not care about conversions, because it is impossible to replicate all of them anyway. So there are a couple of limitations, but it is possible to find workarounds if necessary.