Question

Can I use forward declaration for a class in order to put it’s definition and Implementation later in the program after it’s been used (similar to what is done about functions)?

(I need to join multiple source files of a program into a file, and i want to put the classes’ definitions and Implementations at the end of the file in order to main be at the top of the file.)

Answer 1

Yes you can, to a certain extent.

You have to realize that the C++ compiler is quite stupid, and doesn’t read ahead. This is the reason why you have to use function prototypes (among some other reasons).
Now, a function isn’t hard for compiler to resolve. It just looks at the return type of the function, and the types of the parameters of the function, and just assumes that the function is there, without any knowledge about what’s actually inside the function, because it ultimately doesn’t matter at that point.

However, the contents of the class do matter (the compiler needs to know the size of the class for example). But remember about the not reading ahead bit? When you forward define a class, the compiler doesn’t know about what’s in it, and therefore is missing a lot of information about it. How much space does is need to reserve for example?

Therefore, you can forward define classes, but you can’t use them as value types. The only thing you can do with it (before it has been concretely declared), is use pointers to it (and use it as a function return type and template argument, as pointer out by
@Cheersandhth.-Alf).

If the thing you need to use isn’t a pointer, you should probably use headers (read this if you want to learn more about that).