How do you declare an interface in C++?

Question

How do I setup a class that represents an interface? Is this just an abstract base class?

Answer 1

While it's true that virtual is the de-facto standard to define an interface, let's not forget about the classic C-like pattern, which comes with a constructor in C++:

struct IButton
{
    void (*click)(); // might be std::function(void()) if you prefer

    IButton( void (*click_)() )
    : click(click_)
    {
    }
};

// call as:
// (button.*click)();

This has the advantage that you can re-bind events runtime without having to construct your class again (as C++ does not have a syntax for changing polymorphic types, this is a workaround for chameleon classes).

Tips:

• You might inherit from this as a base class (both virtual and non-virtual are permitted) and fill click in your descendant's constructor.
• You might have the function pointer as a protected member and have a public reference and/or getter.
• As mentioned above, this allows you to switch the implementation in runtime. Thus it's a way to manage state as well. Depending on the number of ifs vs. state changes in your code, this might be faster than switch()es or ifs (turnaround is expected around 3-4 ifs, but always measure first.
• If you choose std::function<> over function pointers, you might be able to manage all your object data within IBase. From this point, you can have value schematics for IBase (e.g., std::vector will work). Note that this might be slower depending on your compiler and STL code; also that current implementations of std::function<> tend to have an overhead when compared to function pointers or even virtual functions (this might change in the future).