Question

Is it possible, in C++ or any other compiled language, to add functionality to a class without modifying the header?

In JavaScript I can add a function to an object after the object is created. Is anything even remote possible in C++?

A use example would be for me to provide a user with some routine.o file, and have them extend it, with something like

void routine::NeverBeforeDeclaredFunction() { ... }

This exact example isn’t allowed, but is anything similar? I’ve thought about letting this class have an array of function pointers, and have a user populate that array with their own function. But this doesn’t provide any advantage, such as access to private variables, or access to this.

Answer 1

Is it possible, in C++

No, you cannot do that at runtime nor at compile time in C++. At least not portably without many headaches. You can create a subclass, but then it’s not “the same class”. If the class is a template class, you could provide your own specializations (but then it’s a different type).

C++/CX (a Microsoft language extension to C++) allows for partial classes. This might be a solution to your problem, if you don’t care about portability. Note that it is done at compile-time.

or any other compiled language, to add functionality to a class without modifying the header?

Objective-C allows this through categories (compile time) and through the Objective-C runtime (runtime).