How to Resolve Build Errors Due to Circular Dependency Amongst Classes in C++

Circular dependencies in C++ lead to different types of common errors that restrict you and other developers from writing an efficient C++ program. These dependencies occur when two or more classes reference each other, forming an inclusion loop. In this article, we will discuss circular dependencies, common errors due to circular dependencies, and strategies to resolve these errors among the classes in C++.

Table of Contents:

• Understanding Circular Dependencies in C++
• Common Errors of Circular Dependencies in C++
  • Compilation Errors
  • Recursive Inclusion Leading to Infinite Inclusion Loops
  • Linker Errors
  • Runtime Errors
• Resolving Circular Dependencies in C++
  • Refactor Code Structure
  • Use Forward Declarations
  • Dependency Injection
  • Using Pointers
  • Dependency Inversion Principle (DIP)
• Conclusion

Understanding Circular Dependencies in C++

A circular dependency in C++ occurs when two or more classes reference each other in such a way that creates an inclusion loop. This occurs when the header files include each other directly, which leads to compilation errors.

Example:

Output:

The code shows how a circular dependency error occurs when class A references class B before any of the classes are fully defined.  Also, the forward declaration of B is not sufficient for the compiler to check its size, which results in an incomplete type error for both classes.

Common Errors of Circular Dependencies in C++

Below are the most common errors of Circular Dependencies in C++

1. Compilation Errors

Compilation errors or incomplete type errors occur when a class is referenced before it is defined. This error occurs in circular dependencies where two classes are dependent on each other. Also, when the compiler finds a reference to a class that has not been fully defined yet, it will give an error showing the type is incomplete.

Example:

Output:

The code shows how an incomplete type error is caused by class A while trying to create an object of class B before B is fully defined. Also, the forward declaration of B is not sufficient for the compiler to check its size, which results in a compilation error.

2. Recursive Inclusion Leading to Infinite Inclusion Loops

A recursive inclusion occurs in a C++ program when two or more header files include each other directly or indirectly, and it causes an infinite loop during compilation. Therefore, this can lead to compilation errors because the compiler gets stuck while trying to resolve the circular dependencies without reaching a conclusion.

Example:

Output:

The code shows how the issue of recursive inclusion occurs by trying to include class B in class A by using a header file (“B.h”), and although class A has an instance of B, it is leading to a circular dependency, which is causing a compilation error.

3. Linker Errors

Linker errors in C++ occur when the linker encounters issues when it tries to combine the object files into a final executable file. The common linker errors that occur due to circular dependencies are multiple definitions, which occur when the same symbol arises multiple times in the program, and undefined references, which occur when the linker cannot find a definition for the declared function.

Example:

Output:

The code shows how an undefined reference error occurs when the two classes A and B are defined with a display() method, and the callB() function creates an instance of B while the callA() function is declared but not defined when the main() is called.

4. Runtime Errors

Runtime errors in C++ occur during the execution of the program in many different forms, such as segmentation faults and stack overflows. These errors occur due to infinite recursion in the program, improper memory access, and circular dependencies.

Example:

Output:

The code shows how to define a class A with a recursiveFunction() that calls itself without a base case, which leads to infinite recursion, and when the main() uses this method, it will give a stack overflow due to the excessive memory consumption from the unbounded recursive calls.

Resolving Circular Dependencies in C++

Below are a few strategies to resolve the circular dependencies in C++

1. Refactor Code Structure

Refactoring the code structure means reorganizing the class dependencies to remove unnecessary coupling. One effective way is to use a third class or a manager that mediates interactions between two tightly coupled classes, which avoids circular dependencies.

Example:

Output:

The code shows how a mediator pattern is used in the C++ program to manage the interactions between classes A and B without directly referencing each other by using a Mediator class.

2. Use Forward Declarations

You must use the forward declarations to resolve or break the circular dependencies because it allows a class to reference another class without requiring its full definition at that point. This approach not only helps to break circular dependencies but also speeds up the compilation.

Example:

Output:

The code shows how the forward declarations are used to break the circular dependency by allowing class A to reference a pointer to class B without the need for the full definition initially, also, the pointer helps to enable A to set an instance of B and call its display() method.

3. Dependency Injection

Dependency Injection (DI) is a design pattern that helps you to reduce the tight coupling between classes by passing dependencies from outside instead of creating them inside a class. This approach makes the code more modular and easier to test.

Example:

Output:

The code shows how the dependency injection allows class A to reference a pointer to class B and set it using a setter method, thus helping to break the circular dependency without requiring the full definition.

4. Using Pointers

You should use pointers instead of direct object instances to break the circular dependencies because the pointers only store the memory addresses and not the full definitions of the objects.

Example:

Output:

The code shows how the forward declarations using a pointer are used to avoid circular dependencies by allowing class A to reference class B without the full definition at the point. Also, class A has a pointer to B, which helps to manage the dependency and call the display() method of B after assigning an instance of B.

5. Dependency Inversion Principle (DIP)

The Dependency Inversion Principle (DIP) in C++ is a most important principle that helps to break the circular dependencies and make the code easier to understand. 

The DIP states that:

1. The high-level modules should not be dependent on the low-level modules, and both should be dependent on abstractions.
2. Abstractions should not be dependent on details, and details should be dependent on abstractions.

How does DIP resolve circular dependencies?

The DIP directly introduces an interface or abstract classes in the program instead of using the direct dependencies between the classes. Both higher-level(A) and lower-level(B) classes depend on a common interface(IB), which avoids circular references, thus promoting flexibility and making it easier to swap the implementations.

Example:

Output:

The code shows that the DIP is using an interface(IB), class A is dependent on the interface instead of the concrete class, class B is implementing IB and defining the display() method, and in main(), an instance of B is injected into A which shows that ho the dependency star managed through an abstraction for breaking tightly coupled classes.

Conclusion

As we have discussed above in this article, circular dependencies in C++ can lead to different types of errors, such as compilation, linker, and runtime errors. To avoid or resolve these errors, you must use the discussed strategies, such as code structuring, DIP, and using forward declarations in your code. So, understanding the circular dependencies, the errors it gives, and the approaches to resolving the errors will help you to write an efficient and error-free C++ code.

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