Question

I am sure this problem is asked a lot but I can’t seem to find anything relevant. I have multiple source files in a C++ project. In typical fashion, there are multiple header files with class and function declarations and associated source files with their definitions. The problem is that when I try to use one of my classes defined in another file as a member for a class in a different file, I get compile errors even when uses the #include directive. What fixes the problem is by prototyping (is that the right word?) the class first before declaring it a member. So if ClassA is in one file and ClassB is in another and I want to use a ClassA member in ClassB I must write:

// ClassA.h
class ClassA {
    public: ClassA (void); };

// ClassB.h
class ClassA;    // prototype
class ClassB {
public: ClassA* ca; };

Is this normal? It doesn't matter if I use pointers or instances, I still must prototype them. I have found that I also must prototype structs and enums if they are in separate files. I can't seem to use constants declared using #define, or const across multiple files I get errors that they are undefined so I am not sure how to give them more than file scope. The same goes for typedefs. I am sure there is some easy fix to this that I am not remembering... any help is appreciated!

Answer 1

The word you’re looking for is “declare”, not “prototype”.

In any case, that’s normal. Usually you’d just include the relevant header files:

// ClassB.h

#include "ClassA.h"

class ClassB {
public: ClassA* ca; };

This will cause problems with circular references, though (but in all other cases it’s fine).

Another thing: Don’t forget your include guards, to protect against multiple inclusion of a file. That is, always write header files in the following way:

#ifndef UNIQUE_IDENTIFIER_HERE
#define UNIQUE_IDENTIFIER_HERE

// Rest of header file here.

#endif // ndef UNIQUE_IDENTIFIER_HERE

The UNIQUE_IDENTIFIER_HERE is usually an all-uppercase variant of the header file name, e.g. ‹PROJECTNAME›_‹PATH_TO_HEADER›_‹HEADERNAME›_H. For example, I’m currently working in a project (called “SeqAn”) that has a header file in the path parallel/taskdata.h. The unique identifier I use is thus SEQAN_PARALLEL_TASKDATA_H.