Calling function from a class

How do you call a method that is part of a class in a hooked process? I'm using my injected dll inside the process to call functions, but a lot of them I want to redirect are part of a class, and when you try to call these functions or hook them, they will throw an exception.

I'm using memcpy to redirect functions to those in my library file; 0xE8+offset.

Any help is appreciated, thanks in advance.