What should i use : functors, interfaces or abstract methods when writing an abstraction(compatibility) layer? (D language)

Question

For example: a compatibility layer between scripting objects (like strings, arrays) or scripting engines( eval() ,readFile() etc.).

Answer 1

Interfaces have my vote. That way, as long as you define the interface any developer will be able to write something compatible fairly easily without you having to distribute too much code to them.

Answer 2

Without more context, I'd have to say interfaces as well. Consider that you can represent a function or delegate as an interface with a single method and that abstract classes are just interfaces with some methods potentially already implemented.

That said, it really depends on what you're trying to accomplish. Interfaces lend themselves to cases where you have lots of objects with a common interface but potentially varying implementations. If you are, for example, designing a very simple callback system for plugins (i.e.: let the plugin hook certain events in the host application) then delegates are probably simpler and sufficient for your needs.

Also keep in mind that if you do go with interfaces, you'll probably need some way for the host to instantiate instances. The easiest way to do this is by registering a delegate with the host under some unique name.

Abstract classes are only useful if you want to use interfaces and provide a default implementation of some things. A better solution in that case is to have an actual interface instead, and provide the default implementation as a mixin.