In C or C++, is there a way to extend a class without inheritance?

Question

Is there a way to implement functionality like Class Categories (of Objective-C) or Extension Methods (of C# 3.0) in C and/or C++?

Answer 1

With regard to C#'s extension methods: Not directly. C++ has less need for these things because C++ supports free functions. I've never used Objective-C so I can't comment there.

Answer 2

C++ doesn't have sealed classes or single class inheritance, so in most cases you can subclass the base class. There are creative ways to make a class non-inheritable, but they are few and far in between. In general, C++ doesn't have the problems C# does that gave birth to extension methods.

C is not Object Orientated, so the question doesn't really apply.

Answer 3

Can you use an interface? Extension methods are an easy way to avoid subclassing, but they are rendered semi-useless when proper OO techniques are used. The reason that they are used with Linq so much is so that the VS team did not have to go and update code that would most likely break a lot of legacy applications.

Per MSDN: "In general, we recommend that you implement extension methods sparingly and only when you have to. Whenever possible, client code that must extend an existing type should do so by creating a new type derived from the existing type."

http://msdn.microsoft.com/en-us/library/bb383977.aspx

Answer 4

Not really. It's not the C++ way to treat classes like this.

Amongst others, Meyers argue that it's best to have a small class with the minimal set of operations that make it fully useful. If you want to expand the feature set, you may add an utility namespace (e.g. namespace ClassUtil) that contains non-member utility functions that operate on that minimal class. It's easy to add functions to a namespace from anywhere.

You can check a discussion on the subject here.