Circular Dependency - Example of Circular Dependencies in C++

Example of Circular Dependencies in C++

Implementation of circular dependencies in C/C++ can be a bit tricky, because any class or structure definition must be placed above its usage in the same file. A circular dependency between classes A and B will thus both require the definition of A to be placed above B, and the definition of B to be placed above A, which of course is impossible. A forward declaration trick is therefore needed to accomplish this.

The following example illustrates how this is done.

• File a.h:

#ifndef A_H #define A_H class B; //forward declaration class A { public: B* b; }; #endif //A_H

• File b.h:

#ifndef B_H #define B_H class A; //forward declaration class B { public: A* a; }; #endif //B_H

• File main.cpp:

#include "a.h" #include "b.h" int main { A a; B b; a.b = &b; b.a = &a; }

Note that although a name (e.g. A) can be declared multiple times, such as in forward declarations, it can only be defined once (the One Definition Rule).