🎉 Ultra-simplified explanation to design patterns! 🎉
A topic that can easily make anyone's mind wobble. Here I try to make them stick in to your mind (and maybe mine) by explaining them in the simplest way possible.
This work, "C++ Design Patterns for Humans", is a derivative of "Design Patterns for Humans" by kamranahmedse, used under CC BY 4.0.
Introduction
Design patterns are solutions to recurring problems; guidelines for how to tackle certain problems. They are not classes, packages, or libraries that you can plug into your application and wait for the magic to happen. These are, rather, guidelines on how to tackle certain problems in certain situations.
Design patterns are solutions to recurring problems; guidelines for how to tackle certain problems.
Wikipedia describes design patterns as:
[...] a general reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into source or machine code. It is a description or template for how to solve a problem that can be used in many different situations.
⚠️ Be Careful
- Design patterns are not a silver bullet to all your problems.
- Do not try to force them; bad things are supposed to happen, if done so.
- Keep in mind that design patterns are solutions to problems, not solutions finding problems; so don't overthink.
- If used in a correct place in a correct manner, they can prove to be a savior; or else they can result in a horrible mess of a code.
Types of Design Patterns
Creational Design Patterns
In plain words:
Creational patterns are focused towards how to instantiate an object or group of related objects.
Wikipedia says:
In software engineering, creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation.
🏠 Simple Factory
Overview
Real world example:
Consider, you are building a house and you need doors. You can either put on your carpenter clothes, bring some wood, glue, nails and all the tools required to build the door and start building it in your house or you can simply call the factory and get the built door delivered to you so that you don't need to learn anything about the door making or to deal with the mess that comes with making it.
In plain words:
Simple factory simply generates an instance for client without exposing any instantiation logic to the client.
Wikipedia says:
In object-oriented programming (OOP), a factory is an object for creating other objects – formally a factory is a function or method that returns objects of a varying prototype or class from some method call, which is assumed to be "new".
Programmatic Example
TODO
When To Use
When creating an object is not just a few assignments and involves some logic, it makes sense to put it in a dedicated factory instead of repeating the same code everywhere.
🏭 Factory Method
Overview
Real world example:
Consider the case of a hiring manager. It is impossible for one person to interview for each of the positions. Based on the job opening, she has to decide and delegate the interview steps to different people.
In plain words:
It provides a way to delegate the instantiation logic to child classes.
Wikipedia says:
In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor.
Programmatic Example
TODO
When To Use
Useful when there is some generic processing in a class but the required sub-class is dynamically decided at runtime. Or putting it in other words, when the client doesn't know what exact sub-class it might need.
🔨 Abstract Factory
Overview
Real world example:
Extending our door example from Simple Factory. Based on your needs you might get a wooden door from a wooden door shop, iron door from an iron shop or a PVC door from the relevant shop. Plus you might need a guy with different kind of specialities to fit the door, for example a carpenter for wooden door, welder for iron door etc. As you can see there is a dependency between the doors now, wooden door needs carpenter, iron door needs a welder etc.
In plain words:
A factory of factories; a factory that groups the individual but related/dependent factories together without specifying their concrete classes.
Wikipedia says:
The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes.
Programmatic Example
TODO
When To Use
When there are interrelated dependencies with not-that-simple creation logic involved.
👷 Builder
Overview
Real world example:
Imagine you are at Hardee's and you order a specific deal, lets say, "Big Hardee" and they hand it over to you without any questions; this is the example of simple factory. But there are cases when the creation logic might involve more steps. For example you want a customized Subway deal, you have several options in how your burger is made e.g what bread do you want? what types of sauces would you like? What cheese would you want? etc. In such cases builder pattern comes to the rescue.
In plain words:
Allows you to create different flavors of an object while avoiding constructor pollution. Useful when there could be several flavors of an object. Or when there are a lot of steps involved in creation of an object.
Wikipedia says:
The builder pattern is an object creation software design pattern with the intentions of finding a solution to the telescoping constructor anti-pattern.
Having said that let me add a bit about what telescoping constructor anti-pattern is. At one point or the other we have all seen a constructor like below:
TODO
As you can see; the number of constructor parameters can quickly get out of hand and it might become difficult to understand the arrangement of parameters. Plus this parameter list could keep on growing if you would want to add more options in future. This is called telescoping constructor anti-pattern.
Programmatic Example
TODO
When To Use
When there could be several flavors of an object and to avoid the constructor telescoping. The key difference from the factory pattern is that; factory pattern is to be used when the creation is a one step process while builder pattern is to be used when the creation is a multi step process.
🐑 Prototype
Overview
Real world example:
Remember dolly? The sheep that was cloned! Lets not get into the details but the key point here is that it is all about cloning.
In plain words:
Create object based on an existing object through cloning.
Wikipedia says:
The prototype pattern is a creational design pattern in software development. It is used when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects.
In short, it allows you to create a copy of an existing object and modify it to your needs, instead of going through the trouble of creating an object from scratch and setting it up.
Programmatic Example
TODO
When To Use
When an object is required that is similar to existing object or when the creation would be expensive as compared to cloning.
💍 Singleton
Overview
Real world example:
There can only be one president of a country at a time. The same president has to be brought to action, whenever duty calls. President here is singleton.
In plain words:
Ensures that only one object of a particular class is ever created.
Wikipedia says:
In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.
Singleton pattern is actually considered an anti-pattern and overuse of it should be avoided. It is not necessarily bad and could have some valid use-cases but should be used with caution because it introduces a global state in your application and change to it in one place could affect in the other areas and it could become pretty difficult to debug. The other bad thing about them is it makes your code tightly coupled plus mocking the singleton could be difficult.
Programmatic Example
TODO
When To Use
TODO
Structural Design Patterns
In plain words:
Structural patterns are mostly concerned with object composition or in other words how the entities can use each other. Or yet another explanation would be, they help in answering "How to build a software component?"
Wikipedia says:
In software engineering, structural design patterns are design patterns that ease the design by identifying a simple way to realize relationships between entities.
🔌 Adapter
TODO
🚡 Bridge
TODO
🌿 Composite
TODO
☕ Decorator
TODO
📦 Facade
TODO
🍃 Flyweight
TODO
🎱 Proxy
TODO
Behavioral Design Patterns
In plain words:
It is concerned with assignment of responsibilities between the objects. What makes them different from structural patterns is they don't just specify the structure but also outline the patterns for message passing/communication between them. Or in other words, they assist in answering "How to run a behavior in software component?"
Wikipedia says:
In software engineering, behavioral design patterns are design patterns that identify common communication patterns between objects and realize these patterns. By doing so, these patterns increase flexibility in carrying out this communication.
- Chain of Responsibility
- Command
- Iterator
- Mediator
- Memento
- Observer
- Visitor
- Strategy
- State
- Template Method
🔗 Chain of Responsibility
TODO
👮 Command
TODO
➿ Iterator
TODO
👽 Mediator
TODO
💾 Memento
TODO
😎 Observer
TODO
🏃 Visitor
TODO
💡 Strategy
TODO
💢 State
TODO
📒 Template Method
TODO
🚦 Wrap Up
And that about wraps it up. I will continue to improve this, so you might want to watch/star this repository to revisit. Also, I have plans on writing the same about the architectural patterns, stay tuned for it.
👬 Contribution
- Report issues
- Open pull request with improvements
- Spread the word
License
All content of this file, unless otherwise noted, is licensed as follows:
- All provided source code examples are covered by the MIT License.
- The C++ badge that appears in the banner image was created by Jeremy Kratz and is licensed by The Standard C++ Foundation.
- Other content, including images, is released under the Creative Common Attribution 4.0 International License.