Summary

The Dependency Inversion Principle (DIP) in C++ is a design principle that promotes maintainable, reusable, and extendable code, stating that high-level modules should not depend on low-level modules; instead, both should depend on abstractions, and abstractions should not depend on details but details should depend on abstractions.

Abstract

When designing code structure in high-level languages like C++, maintainability and reusability are key factors. The Dependency Inversion Principle (DIP) is a design principle that improves code quality by adhering to the statement that high-level modules should not depend on low-level modules, instead both should depend on abstractions. The abstractions should not rely on details, but rather the details should depend on abstractions. A hypothetical role-playing game example illustrates the benefits of implementing DIP using a Player and Door classes interacting through an InteractiveObject interface. This promotes extensibility and allows the Player class to interact with other objects without introducing new methods for each interaction.

Bullet points

The Dependency Inversion Principle (DIP) is a SOLID design principle, which states that high-level modules should not depend on low-level modules, both should depend on abstractions, and abstractions should not depend on details.
High-level modules refer to more abstract and complex operations in code requiring one or more low-level modules to perform functions.
A hypothetical RPG example with Player and Door classes demonstrates a straightforward implementation where the Player depends on the Door class for interaction.
Introducing an InteractiveObject interface that the Door class implements decouples the Player and Door, adhering to DIP and improving code maintainability, reusability, and extendibility.
The Player class only needs to know about the InteractiveObject interface, not the implementation, making it more flexible when interacting with other game objects.
Template-based implementation is another alternative to DIP without explicitly declaring the InteractiveObject interface, demonstrating that DIP isn't limited to interfaces.
Benefits of DIP in code include increased maintainability, reusability, and extendibility.
DIP promotes the use of abstractions instead of detailed low-level implementations to improve code quality.
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Dependency Inversion Principle in C++

Writing Maintainable, Reusable, and Extendable Code

When designing code structure in a high-level language such as C++, it is very important to think about factors such as maintainability and reusability of your code. Software developers are nowadays constantly bombarded with new requirements, platforms and changing library versions. A well-designed software should be able to deal with that. Badly designed (and maintained) software tends toward spaghetti code with dependencies all over the place.

Developers often rely on design patterns and design principles to help them design software. Design patterns are tried and proven recipes for common situations. On the other hand, design principles are guidelines for good system design. While design patterns often apply to a specific language or at least a paradigm (such as object-oriented programming), principles tend to be more general.

The Dependency Inversion Principle (DIP) is one of the SOLID design principles. It states:

High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details. Details should depend on abstractions.

High-level modules generally refer to more abstract and complex operations in your code. These operations require one or more low-level modules to perform their function. The straightforward way to implement such complex functionality therefore is to make the high-level modules depend on low-level modules.

Straightforward Implementation

Let’s have a look at an example of a straightforward implementation using two classes in a hypothetical RPG, Player and Door. The desired functionality is to open or close the door once the player interacts with it (depending on its state). The player class may look as follows:

As you can see, it has one relevant method, interactWith. This method calls the toggleOpen method of the Door class. Let us look at the Door class now:

As you can see, the door in the game toggles between a closed and an open state.

Issues with the Implementation

At a first glance, everything looks fine. The player is able to interact with doors in the game. However, extensibility should always be on our mind. In our implementation, we reduced code extensibility and reusability due to the dependence of the Player class on the Door class. What if the player wants to interact with other objects in the game? We will need to write a separate method for every new object.

Implementation Respecting the Dependency Inversion Principle

The key to this problem is to introduce an InteractiveObject interface that the Door class can implement. An interface introduces an abstraction that will help us decouple the player and the door. In that way, we will adhere to the principle that both the high-level module and the low-level module should depend only on abstractions:

This interface declares the method interact that allows interaction with the object. We also refactor the Door class slightly to implement this interface:

The Player class will now only need to know the InteractiveObject interface and won’t care about the implementation of the object. This adheres to the principle that high-level modules should also depend only on abstractions:

You can imagine communication of modules through abstractions like driving a car: you only need to know how to turn on the engine, how to steer and how to make the car go faster or slower. You don’t need to know the intricacies of the engine and the transmission.

Alternative Implementation

To showcase that the dependency inversion principle is not limited to writing interfaces, let us solve the same problem with templates:

You see that in this case we don’t need to explicitly declare the InteractiveObject interface, but we rather rely on a specific contract that the Door class implements (in this case it needs to implement the interact method. If this contract isn’t implemented, the compiler will throw an error. Since the template method doesn’t use dynamic polymorphism it may run a tiny bit faster. Otherwise, the two implementations are almost identical.

Conclusion

As you can see, the dependency inversion principle increases maintainability, reusability and extendibility of our code by making high-level modules depend on abstractions instead of detailed low-level implementations.