Circular dependency

Encyclopedia : C : CI : CIR : Circular dependency

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 automatic garbage collectors 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.

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 ;
#endif //A_H

File b.h:

#ifndef B_H #define B_H class A; //forward declaration class B ; #endif //B_H

File main.cpp:

#include "a.h" #include "b.h" void main()

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 ;

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() 

A::~A

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