Does the function declaration order matter in a header file?

Question

I wonder if the declaration order of a function in a header has an importance.

Let's imagine : I've got two projects that use the same header definition, and the header had to be copied for some obscure reason. And these headers are not the same in terms of declaration function order.

So the header for my first project would be :

class A {
  someFunctionA();
  someFunctionB();
}

and the header in the second project :

class A {
  someFunctionB();
  someFunctionA();
}

And now what will happen if I use the implementation created in the first project in my second project (like a dynamic library or whatever) ?

I'm aware that I should use the same header definition for both project, I was just wondering if the compiler will use some sort of stack for the functions or will reorder them.

Answer 1

Strictly speaking, you have undefined behaviour unless both class definitions are identical (defined as consisting of the same sequence of tokens after preprocessing).

In practice, there probably won't be any issues as long as the memory layout of both classes remains the same. Adding, removing or reordering non-virtual member functions won't affect the layout. Doing that to base classes, data members or virtual member functions will affect the layout, and could lead to horrendously inscrutable bugs.

Since it's impossible to prevent incompatible changes, I'd strongly advise you not to do this. Either find a way to share the same class definition between projects, or fully branch it and maintain each branch separately.