在 C 中实现 vtable 的目的，而不是使用 C++

Question

I've just started to dive into GStreamer framework and I discovered with quite surprise that despite the fact it written in pure C it uses objects and polymorphism. Then I discovered that it's possible to emulate some of C++ features like inheritance in C. It's about implementing a virtual functions table by ourselves. I'm a little bit confused about this idea why would someone need it. If I had to choose between implementing virtual tables in C and use C++ for the project I would definitely use tested and well known implementation of C++ language features. The whole idea seems to me like reinventing the wheel. What are advantages of having macros like in C:

GST_IS_EVENT(obj)
GST_EVENT_CAST(obj)

over in C++:

dynamic_cast<Derived*>(&baseObj);

There are two things that I can think of:

• C is understand relatively good by more people and it's easier to maintain codebase with more number contributors
• C is simpler and code will be easier to maintain

Are there any technical advantages of C with implementing vtable over C++?

Answer 1

The most important reason is, that C library ABI is also the standard library ABI at least on Linux and Windows and Mac. In other words, these libraries can be used from any language (which supports using external libraries).

C++ is different story. For example, there is no standard ABI for C++ vtable. Also, to support methods and function overloading and name spaces, C++ does name mangling for symbols. And then exceptions are yet another thing which affects how functions are called, and that may even need specific compiler switches even if using the same compiler.

It would be possible to just write "C" with C++, and export only extern "C" symbols, but if you do that, you might as well use C to avoid complexity of mixing languages.

Answer 2

Probably because C++ offers many more features than they need for their project.

Maybe their OOP model is not the same as what C++ provides.