Class name does not name a type in C++

Question

I just started programming in C++, and I\'ve tried to create 2 classes where one will contain the other.

File A.h:

#ifndef _A_h
#define          


        

Answer 1

when you define the class A, in A.h, you explicitely say that the class has a member B.

You MUST include "B.h" in "A.h"

Answer 2

The preprocessor inserts the contents of the files A.h and B.h exactly where the include statement occurs (this is really just copy/paste). When the compiler then parses A.cpp, it finds the declaration of class A before it knows about class B. This causes the error you see. There are two ways to solve this:

1. Include B.h in A.h. It is generally a good idea to include header files in the files where they are needed. If you rely on indirect inclusion though another header, or a special order of includes in the compilation unit (cpp-file), this will only confuse you and others as the project gets bigger.

2. If you use member variable of type B in class A, the compiler needs to know the exact and complete declaration of B, because it needs to create the memory-layout for A. If, on the other hand, you were using a pointer or reference to B, then a forward declaration would suffice, because the memory the compiler needs to reserve for a pointer or reference is independent of the class definition. This would look like this:

   class B; // forward declaration        
   class A {
   public:
       A(int id);
   private:
       int _id;
       B & _b;
   };
   

   This is very useful to avoid circular dependencies among headers.

I hope this helps.

Answer 3

The solution to my problem today was slightly different that the other answers here.

In my case, the problem was caused by a missing close bracket (}) at the end of one of the header files in the include chain.

Essentially, what was happening was that A was including B. Because B was missing a } somewhere in the file, the definitions in B were not correctly found in A.

At first I thought I have circular dependency and added the forward declaration B. But then it started complaining about the fact that something in B was an incomplete type. That's how I thought of double checking the files for syntax errors.

Answer 4

Aren't you missing the #include "B.h" in A.h?