Derived classes in C - What is your favorite method?

Question

In my experience in object oriented C programming I have seen two ways to implement derived classes.

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First Method, have a definition of the pa

Answer 1

I have used method #2 before and found it works quite ok:

• you can upcast to the base type anytime if it is the first member in derived type
• instead of dereferencing all the time to get at base members, just keep two pointers: one for the base interface, one for the derived interface

free() on the pointer to the base structure will of course also free up the derived fields, so that isn't an issue...

Also, I find accessing base fields something I tend to do in a polymorphic situation: I only care about those fields in methods that care about the base type. Fields in the derived type are used in methods only interested in the derived type.

Answer 2

The second way has the advantage of typesafety with inherited methods. If you want to have a method foo(struct parent_class bar) and call it with foo((struct parentclass) derived_class), this will work correctly. The C-Standard defines this. Thus, I'd generally prefer method #2. In general, it is guaranteed that casting a structure to its first member will result in a struct containing the data of the first member of the struct, no matter how memory is laid out.

Answer 3

I'm one of the maintainers of a library that uses method 2. Works just as well as method 1, but without any preprocessor trickery. Or it actually works better, since you can have functions that take the base class as argument and you can just cast to the base struct, C guarantees that this works for the first member.

The more interesting question is, how do you do virtual functions? In our case, the struct has pointers to all the functions, and the initialization set them up. It's slightly simpler, but has more space overhead than the "proper way" with a pointer to a shared vtable.

Anyway, I'd prefer to use C++ rather than kludge it with plain C, but politics..