How to Resolve Circular Dependencies in C++

Having circular dependencies in C++ can cause the compiler to complain about having a type that wasn't declared. The solution is simple, that is to forward declare the class. But knowing such problem exists and how to resolve it can save a lot of time debugging. Let's consider this scenario below. A needs B and B needs A.

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#ifndef A_H_ #define A_H_   class B; // forward declare B   class A { public:     A(B* b);     void trigger();     void execute();     virtual ~A();   private:     B* bptr; }; #endif /* A_H_ */

In class A, we forward declare B. If we didn't forward declare B, we would get compilation error. The only catch for using forward declaration is the B must be a pointer type.

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#include "A.h" // make sure to include A #include "B.h" #include <iostream> using namespace std;   A::A(B* b) : bptr(b) { }   void A::trigger() {     bptr->executeA(this); }   void A::execute() {     cout << "Hello" << endl; }   A::~A() { }

Since we forward declare B in A.h header file, we need to include the actual B header file.

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#ifndef B_H_ #define B_H_   #include "A.h"   class B { public:     B();     void executeA(A* a);     virtual ~B(); };   #endif /* B_H_ */

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#include "B.h"   B::B() { }   void B::executeA(A* a) {     a->execute(); }   B::~B() { }

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#include "A.h" #include "B.h" using namespace std;   int main() {     B b;     A a(&b);     a.trigger(); }