Interfaces and covariance problem

Question

I have a particular class that stores a piece of data, which implements an interface:

template
class MyContainer : public Container

        

Answer 1

Have you thought about using CRTP. I find it a good candidate here. Here is a brief demo. It just explains your ++retval problem (if I understood it correctly). You have to change your IInterface definition from pure virtual to CRTP type interface.

template
struct IInterface
{
  Derived& operator ++ ()
  {
    return ++ *(static_cast(this));
  }
};

struct Something : public IInterface
{
  int x;
  Something& operator ++ ()
  {
    ++x;
    return *this;
  }
};

There are some limitations of CRTP, that the template will always follow your IInterface. Which means that if you are passing a Something object to a function like this:

foo(new Something);

Then, foo() should be defined as:

template
void foo(IInterface *p)
{
  //...
  ++(*p);
}

However for your problem, it can be a good fit.