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