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copied!<p>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.</p> <p>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:</p> <pre><code>//#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; } </code></pre> <p>Now read the code. B is the first class the compiler encounters, and it includes an <code>A&</code> member. What is <code>A</code>? The compiler hasn't encountered any definition of <code>A</code> yet, so it issues an error.</p> <p>The solution is to make a forward declaration of A. At some point before the definition of B, add a line <code>class A;</code></p> <p>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.</p>

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