Question

A big reason why I use OOP is to create code that is easily reusable. For that purpose Java style interfaces are perfect. However, when dealing with C++ I really can’t achieve any sort of functionality like interfaces… at least not with ease.

I know about pure virtual base classes, but what really ticks me off is that they force me into really awkward code with pointers. E.g. map<int, Node*> nodes; (where Node is the virtual base class).

This is sometimes ok, but sometimes pointers to base classes are just not a possible solution. E.g. if you want to return an object packaged as an interface you would have to return a base-class-casted pointer to the object.. but that object is on the stack and won’t be there after the pointer is returned. Of course you could start using the heap extensively to avoid this but that’s adding so much more work than there should be (avoiding memory leaks).

Is there any way to achieve interface-like functionality in C++ without have to awkwardly deal with pointers and the heap?? (Honestly for all that trouble and awkardness id rather just stick with C.)

Answer 1

Template MetaProgramming is a pretty cool thing. The basic idea? “Compile time polymorphism and implicit interfaces”, Effective C++. Basically you can get the interfaces you want via templated classes. A VERY simple example:

template <class T>
bool foo( const T& _object )
{
    if ( _object != _someStupidObject && _object > 0 )
        return true;
    return false;
}

So in the above code what can we say about the object T? Well it must be compatible with ‘_someStupidObject’ OR it must be convertible to a type which is compatible. It must be comparable with an integral value, or again convertible to a type which is. So we have now defined an interface for the class T. The book “Effective C++” offers a much better and more detailed explanation. Hopefully the above code gives you some idea of the “interface” capability of templates. Also have a look at pretty much any of the boost libraries they are almost all chalk full of templatization.