A question about exporting a C++ class to C

I've been reading ab开发者_运维知识库out exporting a C++ class so that C can use it. I've followed this good example from Parashift, and a question came out.

To hold the memory allocated to the C++ class, we define the following structure

typedef struct A A;

So that A* can point to the address of the class A. My question is: does it matter of which type is this pointer? For example, couldn't we typedef something like:

typedef double A;

Since it's probably the C++ code that will be responsible for allocating and deallocating memory.

No, you shouldn't use void* by this. A pointer to an incomplete type struct A is a much better technique. This avoids that you mix pointers to different opaque types by accident. If you'd typedef it to void you could assign a pointer of your opaque type to any other data pointer. Probably not what you want.

Using an incomplete type gives you a bit of type safety for no cost at all.

Opaque pointers are best declared as void*.

This is often known as the Pimpl idiom.

You should use the void type for anything which is intended to be opaque like this. Using a typedef of an undefined struct is ok in that its name can give you a sense of what it really points to.