Headers Including Each Other in C++

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By “the macros” I assume you mean the #ifndef include guards?
If so, #includes should definitely go inside. This is one of the major reasons why include guards exists, because otherwise you easily end up with an infinite recursion as you noticed.

Anyway, the problem is that at the time you use the A and B classes (inside the other class), they have not yet been declared. Look at what the code looks like after the #includes have been processed:

//#include "A.h" start
#ifndef A_H_
#define A_H_

//#include "B.h" start
#ifndef B_H_
#define B_H_

//#include "A.h" start
#ifndef A_H_ // A_H_ is already defined, so the contents of the file are skipped at this point
#endif /*A_H_*/

//#include "A.h" end

class B
{
    private:
            A& a;

    public:
            B(A& a) : a(a) {}
 };

#endif /*B_H_*/

//#include "B.h" end

class A
{
    private:
            B b;

    public:
            A() : b(*this) {}
};

#endif /*A_H_*/
//#include "A.h" end

int main()
{
    A a;
}

Now read the code. B is the first class the compiler encounters, and it includes an A& member. What is A? The compiler hasn’t encountered any definition of A yet, so it issues an error.

The solution is to make a forward declaration of A. At some point before the definition of B, add a line class A;

This gives the compiler the necessary information, that A is a class. We don’t know anything else about it yet, but since B only needs to include a reference to it, this is good enough. In the definition of A, we need a member of type B (not a reference), so here the entire definition of B has to be visible. Which it is, luckily.

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  3. C++ Header order [closed]
  4. cmath vs math.h (And similar c-prefixed vs .h extension headers)
  5. Difference between using std::vector and include<vector>
  6. Redefinition C++ Issue, Multiple Header
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  8. Function Recurssion: [duplicate]
  9. How can this code be reversed? (recursion)
  10. Detecting superfluous #includes in C/C++?
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  13. Tail recursion in C++
  14. Is it legal to recurse into main() in C++? [duplicate]
  15. Creating your own header file in C
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  18. C++ Header Files, Code Separation
  19. *.h or *.hpp for your class definitions
  20. Understanding Recursion to generate permutations
  21. How to tell where a header file is included from?
  22. Can I write C++ code without headers (repetitive function declarations)?
  23. Circular C++ Header Includes
  24. When can you omit the file extension in an #include directive?
  25. Creating N nested for-loops
  26. What is the difference between a .cpp file and a .h file?
  27. What’s time complexity of this algorithm for finding all combinations?
  28. sizeof an array passed as function argument [duplicate]
  29. Do you know tool building tree of include files in project\file?
  30. compiler cannot recognize my class in c++ – cyclic dependency

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