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🎉 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 is a derivative of "Design Patterns for Humans" by kamranahmedse, used under CC BY 4.0.

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.

• 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.

• Creational
• Structural
• Behavioral

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
• Factory Method
• Abstract Factory
• Builder
• Prototype
• Singleton

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".

First of all we have a door base class and a wooden door derived class:

class Door
{
  public:
    virtual float getWidth(void) = 0;
    virtual float getHeight(void) = 0;
};

class WoodenDoor : public Door
{
  public:
    WoodenDoor(float width, float height)
        : width_(width), height_(height)
    {
    }

    float getWidth(void)
    {
      return width_;
    }

    float getHeight(void)
    {
      return height_;
    }

  private:
    float width_;
    float height_;
};

Then we have our door factory that makes the door and returns it:

class DoorFactory
{
  public:
    static std::shared_ptr<Door> makeDoor(float width, float height)
    {
      return std::make_shared<WoodenDoor>(width, height);
    }
};

Here is how this can be used:

// Make a door with dimensions 100x200.
std::shared_ptr<Door> door = DoorFactory::makeDoor(100, 200);

std::cout << "width = " << door->getWidth() << std::endl;
// Output: width = 100
std::cout << "height = " << door->getHeight() << std::endl;
// Output: height = 200

// We can use the factory again to make a door with dimensions 50x100.
std::shared_ptr<Door> door2 = DoorFactory::makeDoor(50, 100);

std::cout << "width = " << door2->getWidth() << std::endl;
// Output: width = 50
std::cout << "height = " << door2->getHeight() << std::endl;
// Output: height = 100

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.

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.

Taking our hiring manager example above. First of all we have an interviewer interface and some implementations for it:

class Interviewer
{
  public:
    virtual void askQuestions(void) = 0;
};

class Developer : public Interviewer
{
  public:
    void askQuestions(void)
    {
      std::cout << "Asking about design patterns!" << std::endl;
    }
};

class CommunityExecutive : public Interviewer
{
  public:
    void askQuestions(void)
    {
      std::cout << "Asking about community building!" << std::endl;
    }
};

Now let us create our hiring manager class:

class HiringManager
{
  public:
    void takeInterview(void)
    {
      std::shared_ptr<Interviewer> interviewer = makeInterviewer();
      interviewer->askQuestions();
    }

  protected:
    virtual std::shared_ptr<Interviewer> makeInterviewer(void) = 0;
};

Now any child can extend it and provide the required interviewer:

class DevelopmentManager : public HiringManager
{
  protected:
    std::shared_ptr<Interviewer> makeInterviewer(void)
    {
      return std::make_shared<Developer>();
    }
};

class MarketingManager : public HiringManager
{
  protected:
    std::shared_ptr<Interviewer> makeInterviewer(void)
    {
      return std::make_shared<CommunityExecutive>();
    }
};

Here is how this can be used:

DevelopmentManager developmentManager = DevelopmentManager();
developmentManager.takeInterview(); // Output: Asking about design patterns!

MarketingManager marketingManager = MarketingManager();
marketingManager.takeInterview(); // Output: Asking about community building!

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.

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.

Translating the door example above. First of all we have our door base class and some derivation of it:

class Door
{
  public:
    virtual void getDescription(void) = 0;
};

class WoodenDoor : public Door
{
  public:
    void getDescription(void)
    {
      std::cout << "I am a wooden door." << std::endl;
    }
};

class IronDoor : public Door
{
  public:
    void getDescription(void)
    {
      std::cout << "I am an iron door." << std::endl;
    }
};

Then we have some fitting experts for each door type:

class DoorFittingExpert
{
  public:
    virtual void getDescription(void) = 0;
};

class Welder : public DoorFittingExpert
{
  public:
    void getDescription(void)
    {
      std::cout << "I can only fit iron doors." << std::endl;
    }
};

class Carpenter : public DoorFittingExpert
{
  public:
    void getDescription(void)
    {
      std::cout << "I can only fit wooden doors." << std::endl;

    }
};

Now we have our abstract factory that would let us make family of related objects i.e. wooden door factory would create a wooden door and wooden door fitting expert and iron door factory would create an iron door and iron door fitting expert:

class DoorFactory
{
  public:
    virtual std::shared_ptr<Door> makeDoor(void) = 0;
    virtual std::shared_ptr<DoorFittingExpert> makeFittingExpert(void) = 0;
};

class WoodenDoorFactory : public DoorFactory
{
  public:
    std::shared_ptr<Door> makeDoor(void)
    {
      return std::make_shared<WoodenDoor>();
    }

    std::shared_ptr<DoorFittingExpert> makeFittingExpert(void)
    {
      return std::make_shared<Carpenter>();
    }
};

class IronDoorFactory : public DoorFactory
{
  public:
    std::shared_ptr<Door> makeDoor(void)
    {
      return std::make_shared<IronDoor>();
    }

    std::shared_ptr<DoorFittingExpert> makeFittingExpert(void)
    {
      return std::make_shared<Welder>();
    }
};

Here is how this can be used:

WoodenDoorFactory woodenFactory = WoodenDoorFactory();
std::shared_ptr<Door> door = woodenFactory.makeDoor();
std::shared_ptr<DoorFittingExpert> expert = woodenFactory.makeFittingExpert();

door->getDescription(); // Output: I am a wooden door.
expert->getDescription(); // Output: I can only fit wooden doors.

IronDoorFactory ironFactory = IronDoorFactory();
std::shared_ptr<Door> door2 = ironFactory.makeDoor();
std::shared_ptr<DoorFittingExpert> expert2 = woodenFactory.makeFittingExpert();

door2->getDescription(); // Output: I am an iron door.
expert2->getDescription(); // Output: I can only fit iron doors.

As you can see the wooden door factory has encapsulated the carpenter and the wooden door also iron door factory has encapsulated the iron door and welder. And thus it had helped us make sure that for each of the created door, we do not get a wrong fitting expert.

When there are interrelated dependencies with not-that-simple creation logic involved.

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:

class Burger
{
  public:
    Burger(int patties, bool cheese = true, bool pepperoni = true,
           bool tomato = true, bool lettuce = true);
}

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.

The sane alternative is to use the builder pattern. First of all we have our burger that we want to make:

class Burger
{
  public:
    Burger(BurgerBuilder* builder);
        : patties_(builder->patties), cheese_(builder->cheese),
          pepperoni_(builder->pepperoni), lettuce_(builder->lettuce),
          tomato_(builder->tomato)
    {
    }

    void getDescription(void);
    {
      std::cout << patties_ << " patties";
      if (cheese_) {
        std::cout << ", cheese";
      }
      if (pepperoni_) {
        std::cout << ", pepperoni";
      }
      if (lettuce_){
        std::cout << ", lettuce";
      }
      if (tomato_) {
        std::cout << ", tomato";
      }
      std::cout << std::endl;
    }

  private:
    int patties_;
    bool cheese_;
    bool pepperoni_;
    bool lettuce_;
    bool tomato_;
};

And then we have the builder

class BurgerBuilder
{
  public:
    BurgerBuilder(int patties)
        : patties(patties), cheese(false), pepperoni(false), lettuce(false),
          tomato(false)
    {
    }

    BurgerBuilder& addCheese(void)
    {
      cheese = true;
      return (*this);
    }

    BurgerBuilder& addPepperoni(void)
    {
      pepperoni = true;
      return (*this);
    }

    BurgerBuilder& addLettuce(void)
    {
      lettuce = true;
      return (*this);
    }

    BurgerBuilder& addTomato(void)
    {
      tomato = true;
      return (*this);
    }

    std::shared_ptr<Burger> build(void)
    {
      return std::make_shared<Burger>(this);
    }

    int patties;
    bool cheese;
    bool pepperoni;
    bool lettuce;
    bool tomato;
};

Here is how this can be used:

// One double patty burger with no dairy.
std::shared_ptr<Burger> burger = BurgerBuilder(2).
    addPepperoni().
    addLettuce().
    addTomato().
    build();
burger->getDescription();
// Output: 2 patties, pepperoni, lettuce, tomato

// One triple patty buger with everything.
std::shared_ptr<Burger> burger2 = BurgerBuilder(3).
    addPepperoni().
    addCheese().
    addLettuce().
    addTomato().
    build();
burger2->getDescription();
// Output: 3 patties, cheese, pepperoni, lettuce, tomato

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.

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.

Let's create a sheep class

class Sheep
{
  public:
    Sheep(const std::string& name, const std::string& category)
        : name_(name), category_(category)
    {
    }

    void setName(const std::string name)
    {
      name_ = name;
    }

    std::string getName(void)
    {
      return name_;
    }

    void setCategory(const std::string category)
    {
      category_ = category;
    }

    std::string getCategory(void)
    {
      return category_;
    }

  private:
    std::string name_;
    std::string category_;
};

Here is how we can clone this object

Sheep original = Sheep("Molly", "Mountain Sheep");
std::cout << original.getName() << std::endl; // Output: Molly
std::cout << original.getCategory() << std::endl; // Output: Mountain Sheep

Sheep clone = original;
clone.setName("Dolly");
std::cout << clone.getName() << std::endl; // Output: Dolly
std::cout << clone.getCategory() << std::endl; // Output: Mountain Sheep

When an object is required that is similar to existing object or when the creation would be expensive as compared to cloning.

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.

To create a singleton, make the constructor private, disable cloning, and create a static variable to house the instance.

class President
{
  public:
    static President& getInstance()
    {
      static President instance;
      return instance;
    }

  private:
    President()
    {
    }

    ~President()
    {
    }
};

Here is how this can be used:

President& president1 = President::getInstance();
President& president2 = President::getInstance();

// There can only be 1 president, so they must be the same.
assert(&president1 == &president2);

When exactly one object is needed to coordinate actions across a system.

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
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

Real world example:

Consider that you have some pictures in your memory card and you need to transfer them to your computer. In order to transfer them you need some kind of adapter that is compatible with your computer ports so that you can attach memory card to your computer. In this case card reader is an adapter.

Another real world example:

Another example would be the famous power adapter; a three legged plug can't be connected to a two pronged outlet, it needs to use a power adapter that makes it compatible with the two pronged outlet.

And another:

Yet another example would be a translator translating words spoken by one person to another.

In plain words:

Adapter pattern lets you wrap an otherwise incompatible object in an adapter to make it compatible with another class.

Wikipedia says:

In software engineering, the adapter pattern is a software design pattern that allows the interface of an existing class to be used as another interface. It is often used to make existing classes work with others without modifying their source code.

Consider a game where there is a hunter and he hunts lions. First we have a lion class that all types of lions have to implement.

class Lion
{
  public:
    virtual void roar(void) = 0;
};

class AfricanLion : public Lion
{
  public:
    void roar(void)
    {
      std::cout << "*African lion roar*" << std::endl;
    }
};

class AsianLion : public Lion
{
  public:
    void roar(void)
    {
      std::cout << "*Asian lion roar*" << std::endl;
    }
};

And hunter expects any implementation of lion class.

class Hunter
{
  public:
    void hunt(Lion& lion)
    {
      lion.roar();
    }
};

Now let's say we have to add a wild dog in our game so that hunter can hunt that also (Note: I do not condone the hunting of any dogs). But we can't do that directly because dog has a different interface. To make it compatible for our hunter, we will have to create an adapter that is compatible.

class WildDog
{
  public:
    void bark(void)
    {
      std::cout << "*wild dog bark*" << std::endl;
    }
};

class WildDogAdapter : public Lion
{
  public:
    WildDogAdapter(std::shared_ptr<WildDog> dog)
        : dog_(dog)
    {
    }

    void roar(void)
    {
      dog_->bark();
    }

  private:
    std::shared_ptr<WildDog> dog_;
};

Here is how this can be used:

std::shared_ptr<WildDog> wildDog = std::make_shared<WildDog>();
WildDogAdapter wildDogAdapter(wildDog);

Hunter hunter;
hunter.hunt(wildDogAdapter); // Output: *wild dog bark*

When you need to make an object compatible with the interface of another class.

Real world example:

Consider you have a website with different pages and you are supposed to allow the user to change the theme. What would you do? Create multiple copies of each of the pages for each of the themes or would you just create separate theme and load them based on the user's preferences? Bridge pattern allows you to do the second i.e.

In plain words:

Bridge pattern is about preferring composition over inheritance. Implementation details are pushed from a hierarchy to another object with a separate hierarchy.

Wikipedia says:

The bridge pattern is a design pattern used in software engineering that is meant to "decouple an abstraction from its implementation so that the two can vary independently."

Translating our WebPage example from above. Here we have the web page hierarchy.

class WebPage
{
  public:
    virtual std::string getContent(void) = 0;
};

class About : public WebPage
{
  public:
    About(std::shared_ptr<Theme> theme)
        : theme_(theme)
    {
    }

    std::string getContent(void)
    {
      return "About page in " + theme_->getColor();
    }

  private:
    std::shared_ptr<Theme> theme_;
};

class Projects : public WebPage
{
  public:
    Projects(std::shared_ptr<Theme> theme)
        : theme_(theme)
    {
    }

    std::string getContent(void)
    {
      return "Projects page in " + theme_->getColor();
    }

  private:
    std::shared_ptr<Theme> theme_;
};


class Careers : public WebPage
{
  public:
    Careers(std::shared_ptr<Theme> theme)
        : theme_(theme)
    {
    }

    std::string getContent(void)
    {
      return "Careers page in " + theme_->getColor();
    }

  private:
    std::shared_ptr<Theme> theme_;
};

And the separate theme hierarchy.

class Theme
{
  public:
    virtual std::string getColor(void) = 0;
};

class DarkTheme : public Theme
{
  public:
    std::string getColor(void)
    {
      return "dark palette";
    }
};

class LightTheme : public Theme
{
  public:
    std::string getColor(void)
    {
      return "light palette";
    }
};

class AquaTheme : public Theme
{
  public:
    std::string getColor(void)
    {
      return "aqua palette";
    }
};

Here is how this can be used:

std::shared_ptr<Theme> darkTheme = std::make_shared<DarkTheme>();
About about(darkTheme);
Careers careers(darkTheme);

std::cout << about.getContent() << std::endl;
// Output: About page in dark palette
std::cout << careers.getContent() << std::endl;
// Output: Careers page in dark palette

When you need to implement several variants of some functionality for a class, you find yourself extending a class in competing ways, or you need to allow switching between implementations at runtime.

Real world example:

Every organization is composed of employees. Each of the employees has the same features i.e. has a salary, has some responsibilities, may or may not report to someone, may or may not have some subordinates etc.

In plain words:

Composite pattern lets clients treat the individual objects in a uniform manner.

Wikipedia says:

In software engineering, the composite pattern is a partitioning design pattern. The composite pattern describes that a group of objects is to be treated in the same way as a single instance of an object. The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.

Taking our employees example from above. Here we have different employee types

class Employee
{
  public:
    virtual std::string getName(void) = 0;
    virtual void setSalary(float salary) = 0;
    virtual float getSalary(void) = 0;
    virtual std::string getRole(void) = 0;
};

class Developer : public Employee
{
  public:
    Developer(const std::string& name, float salary)
        : name_(name), salary_(salary), role_("Developer")
    {
    }

    std::string getName(void)
    {
      return name_;
    }

    void setSalary(float salary)
    {
      salary_ = salary;
    }

    float getSalary(void)
    {
      return salary_;
    }

    std::string getRole(void)
    {
      return role_;
    }

  private:
    std::string name_;
    float salary_;
    std::string role_;
};

class Designer : public Employee
{
  public:
    Designer(const std::string& name, float salary)
        : name_(name), salary_(salary), role_("Designer")
    {
    }

    std::string getName(void)
    {
      return name_;
    }

    void setSalary(float salary)
    {
      salary_ = salary;
    }

    float getSalary(void)
    {
      return salary_;
    }

    std::string getRole(void)
    {
      return role_;
    }

  private:
    std::string name_;
    float salary_;
    std::string role_;
};

Then we have an organization which consists of several different types of employees

class Organization
{
  public:
    void addEmployee(std::shared_ptr<Employee> employee)
    {
      employees_.push_back(employee);
    }

    float getNetSalaries(void)
    {
      float net = 0;
      for (auto employee : employees_) {
        net += employee->getSalary();
      }

      return net;
    }

  private:
    std::vector<std::shared_ptr<Employee>> employees_;
};

Here is how this can be used:

// Prepare the employees.
std::shared_ptr<Employee> jane = std::make_shared<Developer>("Jane", 50000);
std::shared_ptr<Employee> john = std::make_shared<Designer>("John", 45000);

// Add them to the organization.
Organization org;
org.addEmployee(jane);
org.addEmployee(john);

// Get the net salaries.
std::cout << org.getNetSalaries() << std::endl; // Output: 95000

TODO

Real world example:

Imagine you run a car service shop offering multiple services. Now how do you calculate the bill to be charged? You pick one service and dynamically keep adding to it the prices for the provided services till you get the final cost. Here each type of service is a decorator.

In plain words:

Decorator pattern lets you dynamically change the behavior of an object at run time by wrapping them in an object of a decorator class.

Wikipedia says:

In object-oriented programming, the decorator pattern is a design pattern that allows behavior to be added to an individual object, either statically or dynamically, without affecting the behavior of other objects from the same class. The decorator pattern is often useful for adhering to the Single Responsibility Principle, as it allows functionality to be divided between classes with unique areas of concern.

Lets take coffee for example. First of all we have a simple coffee implementing the coffee class

class Coffee
{
  public:
    virtual float getPrice(void) = 0;
    virtual std::string getDescription(void) = 0;
};

class SimpleCoffee : public Coffee
{
  public:
    float getPrice(void)
    {
      return 3;
    }

    std::string getDescription(void)
    {
      return "Simple coffee";
    }
};

We want to make the code extensible to allow options to modify it if required. Lets make some add-ons (decorators)

class MilkCoffee : public Coffee
{
  public:
    MilkCoffee(std::shared_ptr<Coffee> coffee)
        : coffee_(coffee)
    {
    }

    float getPrice(void)
    {
      return coffee_->getPrice() + 0.5;
    }

    std::string getDescription(void)
    {
      return coffee_->getDescription() + ", milk";
    }

  private:
    std::shared_ptr<Coffee> coffee_;
};

class WhipCoffee : public Coffee
{
  public:
    WhipCoffee(std::shared_ptr<Coffee> coffee)
        : coffee_(coffee)
    {
    }

    float getPrice(void)
    {
      return coffee_->getPrice() + 2;
    }

    std::string getDescription(void)
    {
      return coffee_->getDescription() + ", whip";
    }

  private:
    std::shared_ptr<Coffee> coffee_;
};

class VanillaCoffee : public Coffee
{
  public:
    VanillaCoffee(std::shared_ptr<Coffee> coffee)
        : coffee_(coffee)
    {
    }

    float getPrice(void)
    {
      return coffee_->getPrice() + 1;
    }

    std::string getDescription(void)
    {
      return coffee_->getDescription() + ", vanilla";
    }

  private:
    std::shared_ptr<Coffee> coffee_;
};

Here is how this can be used:

std::shared_ptr<Coffee> simple = std::make_shared<SimpleCoffee>();
std::cout << simple->getPrice() << std::endl;
// Output: 3
std::cout << simple->getDescription() << std::endl;
// Output: Simple coffee

std::shared_ptr<Coffee> milk = std::make_shared<MilkCoffee>(simple);
std::cout << milk->getPrice() << std::endl;
// Output: 3.5
std::cout << milk->getDescription() << std::endl;
// Output: Simple coffee, milk

std::shared_ptr<Coffee> whip = std::make_shared<WhipCoffee>(milk);
std::cout << whip->getPrice() << std::endl;
// Output: 5.5
std::cout << whip->getDescription() << std::endl;
// Output: Simple coffee, milk, whip

std::shared_ptr<Coffee> vanilla = std::make_shared<VanillaCoffee>(whip);
std::cout << vanilla->getPrice() << std::endl;
// Output: 6.5
std::cout << vanilla->getDescription() << std::endl;
// Output: Simple coffee, milk, whip, vanilla

TODO

Real world example:

How do you turn on the computer? "Hit the power button" you say! That is what you believe because you are using a simple interface that computer provides on the outside, internally it has to do a lot of stuff to make it happen. This simple interface to the complex subsystem is a facade.

In plain words:

Facade pattern provides a simplified interface to a complex subsystem.

Wikipedia says:

A facade is an object that provides a simplified interface to a larger body of code, such as a class library.

Taking our computer example from above. Here we have the computer class

class Computer
{
  public:
    void makeBootSound(void)
    {
      std::cout << "Beep!" << std::endl;
    }

    void showLoadingScreen(void)
    {
      std::cout << "Loading..." << std::endl;
    }

    void showWelcomeScreen(void)
    {
      std::cout << "Ready to use!" << std::endl;
    }

    void closeEverything(void)
    {
      std::cout << "Closing all programs!" << std::endl;
    }
    void sleep(void)
    {
      std::cout << "Zzz" << std::endl;
    }
};

Here we have the facade

class ComputerFacade
{
  public:
    ComputerFacade(std::shared_ptr<Computer> computer)
        : computer_(computer)
    {
    }

    void turnOn(void)
    {
      computer_->makeBootSound();
      computer_->showLoadingScreen();
      computer_->showWelcomeScreen();
    }

    void turnOff(void)
    {
      computer_->closeEverything();
      computer_->sleep();
    }

  private:
    std::shared_ptr<Computer> computer_;
};

Here is how this can be used:

std::shared_ptr<Computer> computer = std::make_shared<Computer>();
ComputerFacade facade(computer);

facade.turnOn();
// Output:
// Beep!
// Loading...
// Ready to use!

facade.turnOff();
// Output:
// Closing all programs!
// Zzz

TODO

Real world example:

Did you ever have fresh tea from some stall? They often make more than one cup that you demanded and save the rest for any other customer so to save the resources e.g. gas etc. Flyweight pattern is all about that i.e. sharing.

In plain words:

It is used to minimize memory usage or computational expenses by sharing as much as possible with similar objects.

Wikipedia says:

In computer programming, flyweight is a software design pattern. A flyweight is an object that minimizes memory use by sharing as much data as possible with other similar objects; it is a way to use objects in large numbers when a simple repeated representation would use an unacceptable amount of memory.

Translating our tea example from above. First of all we have tea types and tea maker

struct Tea
{
};

class TeaMaker
{
  public:
    std::shared_ptr<Tea> make(const std::string& preference)
    {
      auto match = availableTea_.find(preference);
      if (match == availableTea_.end()) {
        availableTea_[preference] = std::make_shared<Tea>();
      }

      return availableTea_[preference];
    }

    int getPreferenceCount(void)
    {
      return availableTea_.size();
    }

  private:
    std::unordered_map<std::string, std::shared_ptr<Tea>> availableTea_;
};

Then we have the tea shop which takes orders and serves them

class TeaShop
{
  public:
    TeaShop(std::shared_ptr<TeaMaker> maker)
        : maker_(maker)
    {
    }

    void takeOrder(const std::string& preference, int table)
    {
      orders_[table] = maker_->make(preference);
    }

    void serve(void)
    {
      for (auto order : orders_) {
        std::cout << "Serving tea to table " << order.first << std::endl;
      }
    }

    int getPreferenceCount(void)
    {
      return maker_->getPreferenceCount();
    }

  private:
    std::shared_ptr<TeaMaker> maker_;
    std::unordered_map<int, std::shared_ptr<Tea>> orders_;
};

Here is how this can be used:

std::shared_ptr<TeaMaker> maker = std::make_shared<TeaMaker>();
TeaShop shop(maker);

// No orders have been taken, so there are no available teas.
std::cout << shop.getPreferenceCount() << std::endl; // Output: 0

// Take some orders.
shop.takeOrder("half sugar", 1);
shop.takeOrder("with milk", 2);
shop.takeOrder("with boba", 5);

// We create a new batch of tea for each preference.
std::cout << shop.getPreferenceCount() << std::endl; // Output: 3

// Take an order with a previously requested preference.
shop.takeOrder("half sugar", 4);

// Since we have some tea from the last time this request was made, no new
// batch was created.
std::cout << shop.getPreferenceCount() << std::endl; // Output: 3

// Serve the customers.
shop.serve();
// Output: (Note: Since the map is unordered, the serving order may vary.)
// Serving tea to table 4
// Serving tea to table 5
// Serving tea to table 1
// Serving tea to table 2

TODO

Real world example:

Have you ever used an access card to go through a door? There are multiple options to open that door i.e. it can be opened either using access card or by pressing a button that bypasses the security. The door's main functionality is to open but there is a proxy added on top of it to add some functionality. Let me better explain it using the code example below.

In plain words:

Using the proxy pattern, a class represents the functionality of another class.

Wikipedia says:

A proxy, in its most general form, is a class functioning as an interface to something else. A proxy is a wrapper or agent object that is being called by the client to access the real serving object behind the scenes. Use of the proxy can simply be forwarding to the real object, or can provide additional logic. In the proxy extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked.

Taking our security door example from above. Firstly we have the door class and an implementation of door

class Door
{
  public:
    virtual void open(void) = 0;
    virtual void close(void) = 0;
};

class LabDoor : public Door
{
  public:
    void open(void)
    {
      std::cout << "Opening lab door" << std::endl;
    }

    void close(void)
    {
      std::cout << "Closing lab door" << std::endl;
    }
};

Then we have a proxy to secure any doors that we want

class SecuredDoor
{
  public:
    SecuredDoor(std::shared_ptr<Door> door)
        : door_(door)
    {
    }

    void open(const std::string& password)
    {
      if (authenticate(password)) {
        door_->open();
      } else {
        std::cout << "No way, Jose!" << std::endl;
      }
    }

    void close(void)
    {
      door_->close();
    }

  private:
    bool authenticate(const std::string& password)
    {
      return password == "Bond007";
    }

    std::shared_ptr<Door> door_;
};

Here is how this can be used:

std::shared_ptr<Door> labDoor = std::make_shared<LabDoor>();
SecuredDoor securedDoor(labDoor);

securedDoor.open("invalid"); // Output: No way, Jose!
securedDoor.open("Bond007"); // Output: Opening lab door
securedDoor.close(); // Output: Closing lab door

TODO

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

Real world example:

For example, you have three payment methods (A, B and C) setup in your account; each having a different amount in it. A has 100 USD, B has 300 USD and C having 1000 USD and the preference for payments is chosen as A then B then C. You try to purchase something that is worth 210 USD. Using Chain of Responsibility, first of all account A will be checked if it can make the purchase, if yes purchase will be made and the chain will be broken. If not, request will move forward to account B checking for amount if yes chain will be broken otherwise the request will keep forwarding till it finds the suitable handler. Here A, B and C are links of the chain and the whole phenomenon is Chain of Responsibility.

In plain words:

It helps building a chain of objects. Request enters from one end and keeps going from object to object till it finds the suitable handler.

Wikipedia says:

In object-oriented design, the chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects. Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain.

Translating our account example above. First of all we have a base account having the logic for chaining the accounts together and some accounts.

class Account
{
  public:
    Account(void)
        : name_("empty account"), balance_(0), successor_()
    {
    }

    void setNext(std::shared_ptr<Account> account)
    {
      successor_ = account;
    }

    void pay(float amount)
    {
      if (canPay(amount)) {
        std::cout << "Paid " << amount << " using " << name_ << "."
                  << std::endl;
        balance_ -= amount;
      } else if (successor_) {
        std::cout << "Cannot pay using " << name_ << ". Proceeding ..."
                  << std::endl;
        successor_->pay(amount);
      } else {
        std::cerr << "None of the accounts have enough balance." << std::endl;
      }
    }

    bool canPay(float amount)
    {
      return balance_ >= amount;
    }

  protected:
    std::string name_;
    float balance_;
    std::shared_ptr<Account> successor_;
};

class Bank : public Account
{
  public:
    Bank(float balance)
    {
      name_ = "bank";
      balance_ = balance;
    }
};

class Paypal : public Account
{
  public:
    Paypal(float balance)
    {
      name_ = "paypal";
      balance_ = balance;
    }
};

class Bitcoin : public Account
{
  public:
    Bitcoin(float balance)
    {
      name_ = "bitcoin";
      balance_ = balance;
    }
};

Now let's prepare the chain using the links defined above (i.e., Bank, Paypal, Bitcoin).

// We are going to create the chain: bank->paypal->bitcoin.

// First, create the accounts.
std::shared_ptr<Bank> bank = std::make_shared<Bank>(100);
std::shared_ptr<Paypal> paypal = std::make_shared<Paypal>(200);
std::shared_ptr<Bitcoin> bitcoin = std::make_shared<Bitcoin>(300);

// Next, establish the order.
bank->setNext(paypal);
paypal->setNext(bitcoin);

// Let's try to pay using the first priority (i.e., the bank).
bank->pay(250);
// Output:
// Cannot pay using bank. Proceeding ...
// Cannot pay using paypal. Proceeding ...
// Paid 250 using bitcoin.

TODO

Real world example:

A generic example would be you ordering food at a restaurant. You (i.e. Client) ask the waiter (i.e. Invoker) to bring some food (i.e. Command) and waiter simply forwards the request to Chef (i.e. Receiver) who has the knowledge of what and how to cook.

Another example:

Another example would be you (i.e. Client) switching on (i.e. Command) the television (i.e. Receiver) using a remote control (Invoker).

In plain words:

Allows you to encapsulate actions in objects. The key idea behind this pattern is to provide the means to decouple client from receiver.

Wikipedia says:

In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time. This information includes the method name, the object that owns the method and values for the method parameters.

First of all we have the receiver that has the implementation of every action that could be performed.

// The receiver.
class Bulb
{
  public:
    void turnOn(void)
    {
      std::cout << "Bulb has been lit." << std::endl;
    }

    void turnOff(void)
    {
      std::cout << "Darkness!" << std::endl;
    }
};

Then we have an interface that each of the commands are going to implement and then we have a set of commands.

class Command
{
  public:
    virtual void execute(void) = 0;
    virtual void undo(void) = 0;
    virtual void redo(void) = 0;
};

// A command.
class TurnOn : public Command
{
  public:
    TurnOn(std::shared_ptr<Bulb> bulb)
        : bulb_(bulb)
    {
    }

    void execute(void)
    {
      bulb_->turnOn();
    }

    void undo(void)
    {
      bulb_->turnOff();
    }

    void redo(void)
    {
      execute();
    }

  private:
    std::shared_ptr<Bulb> bulb_;
};

// Another command.
class TurnOff : public Command
{
  public:
    TurnOff(std::shared_ptr<Bulb> bulb)
        : bulb_(bulb)
    {
    }

    void execute(void)
    {
      bulb_->turnOff();
    }

    void undo(void)
    {
      bulb_->turnOn();
    }

    void redo(void)
    {
      execute();
    }

  private:
    std::shared_ptr<Bulb> bulb_;
};

Then we have an invoker with whom the client will interact to process any commands.

// The invoker.
class RemoteControl
{
  public:
    void submit(std::shared_ptr<Command> command)
    {
      command->execute();
    }
};

Here is how this can be used:

std::shared_ptr<Bulb> bulb = std::make_shared<Bulb>();

std::shared_ptr<TurnOn> turnOn = std::make_shared<TurnOn>(bulb);
std::shared_ptr<TurnOff> turnOff = std::make_shared<TurnOff>(bulb);

RemoteControl remote;
remote.submit(turnOn);
remote.submit(turnOff);
// Output:
// Bulb has been lit.
// Darkness!

Command pattern can also be used to implement a transaction based system. Where you keep maintaining the history of commands as soon as you execute them. If the final command is successfully executed, all good otherwise just iterate through the history and keep executing the undo on all the executed commands.

TODO

Real world example:

An old radio set will be a good example of iterator, where user could start at some channel and then use next or previous buttons to go through the respective channels. Or take an example of MP3 player or a TV set where you could press the next and previous buttons to go through the consecutive channels or in other words they all provide an interface to iterate through the respective channels, songs or radio stations.

In plain words:

It presents a way to access the elements of an object without exposing the underlying presentation.

Wikipedia says:

In object-oriented programming, the iterator pattern is a design pattern in which an iterator is used to traverse a container and access the container's elements. The iterator pattern decouples algorithms from containers; in some cases, algorithms are necessarily container-specific and thus cannot be decoupled.

TODO

TODO

Real world example:

A general example would be when you talk to someone on your mobile phone, there is a network provider sitting between you and them and your conversation goes through it instead of being directly sent. In this case network provider is mediator.

In plain words:

Mediator pattern adds a third party object (called mediator) to control the interaction between two objects (called colleagues). It helps reduce the coupling between the classes communicating with each other. Because now they don't need to have the knowledge of each other's implementation.

Wikipedia says:

In software engineering, the mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program's running behavior.

TODO

TODO

Real world example:

Take the example of calculator (i.e. originator), where whenever you perform some calculation the last calculation is saved in memory (i.e. memento) so that you can get back to it and maybe get it restored using some action buttons (i.e. caretaker).

In plain words:

Memento pattern is about capturing and storing the current state of an object in a manner that it can be restored later on in a smooth manner.

Wikipedia says:

The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).

Usually useful when you need to provide some sort of undo functionality.

TODO

TODO

Real world example:

A good example would be the job seekers where they subscribe to some job posting site and they are notified whenever there is a matching job opportunity.

In plain words:

Defines a dependency between objects so that whenever an object changes its state, all its dependents are notified.

Wikipedia says:

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

TODO

TODO

Real world example:

Consider someone visiting Dubai. They just need a way (i.e. visa) to enter Dubai. After arrival, they can come and visit any place in Dubai on their own without having to ask for permission or to do some leg work in order to visit any place here; just let them know of a place and they can visit it. Visitor pattern lets you do just that, it helps you add places to visit so that they can visit as much as they can without having to do any legwork.

In plain words:

Visitor pattern lets you add further operations to objects without having to modify them.

Wikipedia says:

In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures. It is one way to follow the open/closed principle.

TODO

TODO

Real world example:

Consider the example of sorting, we implemented bubble sort but the data started to grow and bubble sort started getting very slow. In order to tackle this we implemented Quick sort. But now although the quick sort algorithm was doing better for large datasets, it was very slow for smaller datasets. In order to handle this we implemented a strategy where for small datasets, bubble sort will be used and for larger, quick sort.

In plain words:

Strategy pattern allows you to switch the algorithm or strategy based upon the situation.

Wikipedia says:

In computer programming, the strategy pattern (also known as the policy pattern) is a behavioural software design pattern that enables an algorithm's behavior to be selected at runtime.

TODO

TODO

Real world example:

Imagine you are using some drawing application, you choose the paint brush to draw. Now the brush changes its behavior based on the selected color i.e. if you have chosen red color it will draw in red, if blue then it will be in blue etc.

In plain words:

It lets you change the behavior of a class when the state changes.

Wikipedia says:

The state pattern is a behavioral software design pattern that implements a state machine in an object-oriented way. With the state pattern, a state machine is implemented by implementing each individual state as a derived class of the state pattern interface, and implementing state transitions by invoking methods defined by the pattern's superclass. The state pattern can be interpreted as a strategy pattern which is able to switch the current strategy through invocations of methods defined in the pattern's interface.

TODO

TODO

Real world example:

Suppose we are getting some house built. The steps for building might look like:

• Prepare the base of house
• Build the walls
• Add roof
• Add other floors

The order of these steps could never be changed i.e. you can't build the roof before building the walls etc but each of the steps could be modified for example walls can be made of wood or polyester or stone.

In plain words:

Template method defines the skeleton of how a certain algorithm could be performed, but defers the implementation of those steps to the children classes.

Wikipedia says:

In software engineering, the template method pattern is a behavioral design pattern that defines the program skeleton of an algorithm in an operation, deferring some steps to subclasses. It lets one redefine certain steps of an algorithm without changing the algorithm's structure.

TODO

TODO

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.

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• All provided source code examples are covered by the MIT License.
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