为什么我不能在未链接的DLL上调用方法，但可以做到这一点？ C ++

Question

So I've got DLL Alice and DLL Bob. I cannot call each others methods. But there's one bridge. I can assign a pointer (I'll say the pointer from now on) from either DLL and read it in either DLL. But if I assign the pointer to be an instance of a class defined in Alice, I cannot call it's methods from Bob and vice versa. I also cannot access it's members. They are simply not linked (I get LNK error). So far so good.

And now, to the question. I've created the same class (just different name - let's call it MirrorClass) in Alice and in Bob. When I assign the pointer (both DLLs can access it) an instance of the MirrorClass created in Alice, I still cannot call it's methods from Bob. But what I can do is pretend (explicit casting) it's an instance from Bob's MirrorClass and it'll call Bob's methods defined in Bob's version of MirrorClass and everything still works. Even though it was Alice's instance of MirrorClass. Obviously once I change the layout of MirrorClass, it stops working.

How is that even possible ? I guess it has something to do with virtual addresses maybe, or perhaps every DLL has it's own address space. But then, objects are just pointers, so from my understanding you wouldn't even need linking, so I'm really off with my understanding of this.

Thanks for every advice.

Answer 1

If you have a pointer to a class, to use the methods the class vtbl must be defined (for example, using an abstract class (pure virtual functions)). That is how COM works in Windows, for example.

Now If you got the same interface in both DLLs and the same compiler build, the first vtbl would be the same with the second, which make hard-casting from A* to B* to work.

This is of course UB. It may or it may not work.

The DLLs share the same address space when loaded into the same process, and each one can use any of the other.