Circular dependencies is a situation which occurs in object oriented programming when two or more objects point towards each other in a circular fashion.

Circular dependencies are usually used to implement callback functionality in programs. However, occurrences of circular dependencies should normally be limited, since they make the resulting object-models unclear and unstructured, and are hence considered a bad programming habit. Circular dependencies will also prevent some very primitive automatic garbage collectors (those that use reference counting) from deallocating objects.

Circular dependencies in C++

Implementation of circular dependencies in C/C++ can be a bit tricky, due to the requirement that any class or structure definition must be placed above their 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. To meet Wikipedias quality standards, this article or section may require cleanup. ...

Circular dependencies in C/C++ are implemented by using forward declarations. 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" void main() { A a; B b; a.b = &b; b.a = &a; } 

Self-reference example

Following is another example of forward declaration, which might be useful if the application needs a self-sustaining array of objects which is able to add and remove objects from itself during run-time:

• File a.h:

 class A { public: static A *first, *last; A *previous, *next; A(); ~A(); }; 

The static variables first and last have to be defined, because their declaration does not reserve memory space for them. Note: static variables do not change from object to object and stay the same for this given class.

• File a.cpp:

 #include "a.h" A *A::first, *A::last; // don't put the word static here, that will cause an error A::A() { previous=last; previous->next=this; last=this; next=0; } A::~A { previous->next=next; next->previous=previous; }