---
title: "Prototype"
type: "design-pattern"
slug: "prototype"
url: "http://localhost:3000/en/design-patterns/prototype.md"
category: "Creational Patterns"
description: "Prototype is a creational design pattern that lets you copy existing objects without making your code dependent on their classes."
languages: ["java", "csharp", "cpp", "go", "php", "python", "ruby", "rust", "swift", "typescript"]
---
# Prototype

> Prototype is a creational design pattern that lets you copy existing objects without making your code dependent on their classes.

## Intent

**Prototype** is a creational design pattern that lets you copy existing objects without making your code dependent on their classes.

## Problem

Say you have an object, and you want to create an exact copy of it. How would you do it? First, you have to create a new object of the same class. Then you have to go through all the fields of the original object and copy their values over to the new object.

Nice! But there’s a catch. Not all objects can be copied that way because some of the object’s fields may be private and not visible from outside of the object itself.

Copying an object “from the outside” [isn’t](/antipatterns/cargo-cult-programming) always possible.

There’s one more problem with the direct approach. Since you have to know the object’s class to create a duplicate, your code becomes dependent on that class. If the extra dependency doesn’t scare you, there’s another catch. Sometimes you only know the interface that the object follows, but not its concrete class, when, for example, a parameter in a method accepts any objects that follow some interface.

## Solution

The Prototype pattern delegates the cloning process to the actual objects that are being cloned. The pattern declares a common interface for all objects that support cloning. This interface lets you clone an object without coupling your code to the class of that object. Usually, such an interface contains just a single `clone` method.

The implementation of the `clone` method is very similar in all classes. The method creates an object of the current class and carries over all of the field values of the old object into the new one. You can even copy private fields because most programming languages let objects access private fields of other objects that belong to the same class.

An object that supports cloning is called a _prototype_. When your objects have dozens of fields and hundreds of possible configurations, cloning them might serve as an alternative to subclassing.

Pre-built prototypes can be an alternative to subclassing.

Here’s how it works: you create a set of objects, configured in various ways. When you need an object like the one you’ve configured, you just clone a prototype instead of constructing a new object from scratch.

## Structure

#### Basic implementation

1. The **Prototype** interface declares the cloning methods. In most cases, it’s a single `clone` method.
2. The **Concrete Prototype** class implements the cloning method. In addition to copying the original object’s data to the clone, this method may also handle some edge cases of the cloning process related to cloning linked objects, untangling recursive dependencies, etc.
3. The **Client** can produce a copy of any object that follows the prototype interface.

#### Prototype registry implementation

1. The **Prototype Registry** provides an easy way to access frequently-used prototypes. It stores a set of pre-built objects that are ready to be copied. The simplest prototype registry is a `name → prototype` hash map. However, if you need better search criteria than a simple name, you can build a much more robust version of the registry.

## Pseudocode

In this example, the **Prototype** pattern lets you produce exact copies of geometric objects, without coupling the code to their classes.

Cloning a set of objects that belong to a class hierarchy.

All shape classes follow the same interface, which provides a cloning method. A subclass may call the parent’s cloning method before copying its own field values to the resulting object.

// Base prototype.
abstract class Shape is
    field X: int
    field Y: int
    field color: string

    // A regular constructor.
    constructor Shape() is
        // ...

    // The prototype constructor. A fresh object is initialized
    // with values from the existing object.
    constructor Shape(source: Shape) is
        this()
        this.X = source.X
        this.Y = source.Y
        this.color = source.color

    // The clone operation returns one of the Shape subclasses.
    abstract method clone():Shape

// Concrete prototype. The cloning method creates a new object
// in one go by calling the constructor of the current class and
// passing the current object as the constructor's argument.
// Performing all the actual copying in the constructor helps to
// keep the result consistent: the constructor will not return a
// result until the new object is fully built; thus, no object
// can have a reference to a partially-built clone.
class Rectangle extends Shape is
    field width: int
    field height: int

    constructor Rectangle(source: Rectangle) is
        // A parent constructor call is needed to copy private
        // fields defined in the parent class.
        super(source)
        this.width = source.width
        this.height = source.height

    method clone():Shape is
        return new Rectangle(this)

class Circle extends Shape is
    field radius: int

    constructor Circle(source: Circle) is
        super(source)
        this.radius = source.radius

    method clone():Shape is
        return new Circle(this)

// Somewhere in the client code.
class Application is
    field shapes: array of Shape

    constructor Application() is
        Circle circle = new Circle()
        circle.X = 10
        circle.Y = 10
        circle.radius = 20
        shapes.add(circle)

        Circle anotherCircle = circle.clone()
        shapes.add(anotherCircle)
        // The `anotherCircle` variable contains an exact copy
        // of the `circle` object.

        Rectangle rectangle = new Rectangle()
        rectangle.width = 10
        rectangle.height = 20
        shapes.add(rectangle)

    method businessLogic() is
        // Prototype rocks because it lets you produce a copy of
        // an object without knowing anything about its type.
        Array shapesCopy = new Array of Shapes.

        // For instance, we don't know the exact elements in the
        // shapes array. All we know is that they are all
        // shapes. But thanks to polymorphism, when we call the
        // `clone` method on a shape the program checks its real
        // class and runs the appropriate clone method defined
        // in that class. That's why we get proper clones
        // instead of a set of simple Shape objects.
        foreach (s in shapes) do
            shapesCopy.add(s.clone())

        // The `shapesCopy` array contains exact copies of the
        // `shape` array's children.

## Applicability

Use the Prototype pattern when your code shouldn’t depend on the concrete classes of objects that you need to copy.

 This happens a lot when your code works with objects passed to you from 3rd-party code via some interface. The concrete classes of these objects are unknown, and you couldn’t depend on them even if you wanted to.

The Prototype pattern provides the client code with a general interface for working with all objects that support cloning. This interface makes the client code independent from the concrete classes of objects that it clones.

 Use the pattern when you want to reduce the number of subclasses that only differ in the way they initialize their respective objects.

 Suppose you have a complex class that requires a laborious configuration before it can be used. There are several common ways to configure this class, and this code is scattered through your app. To reduce the duplication, you create several subclasses and put every common configuration code into their constructors. You solved the duplication problem, but now you have lots of dummy subclasses.

The Prototype pattern lets you use a set of pre-built objects configured in various ways as prototypes. Instead of instantiating a subclass that matches some configuration, the client can simply look for an appropriate prototype and clone it.

## How to Implement

1. Create the prototype interface and declare the `clone` method in it. Or just add the method to all classes of an existing class hierarchy, if you have one.
2. A prototype class must define the alternative constructor that accepts an object of that class as an argument. The constructor must copy the values of all fields defined in the class from the passed object into the newly created instance. If you’re changing a subclass, you must call the parent constructor to let the superclass handle the cloning of its private fields.
If your programming language doesn’t support method overloading, you won’t be able to create a separate “prototype” constructor. Thus, copying the object’s data into the newly created clone will have to be performed within the `clone` method. Still, having this code in a regular constructor is safer because the resulting object is returned fully configured right after you call the `new` operator.
3. The cloning method usually consists of just one line: running a `new` operator with the prototypical version of the constructor. Note, that every class must explicitly override the cloning method and use its own class name along with the `new` operator. Otherwise, the cloning method may produce an object of a parent class.
4. Optionally, create a centralized prototype registry to store a catalog of frequently used prototypes.
You can implement the registry as a new factory class or put it in the base prototype class with a static method for fetching the prototype. This method should search for a prototype based on search criteria that the client code passes to the method. The criteria might either be a simple string tag or a complex set of search parameters. After the appropriate prototype is found, the registry should clone it and return the copy to the client.
Finally, replace the direct calls to the subclasses’ constructors with calls to the factory method of the prototype registry.

## Pros

* You can clone objects without coupling to their concrete classes.
* You can get rid of repeated initialization code in favor of cloning pre-built prototypes.
* You can produce complex objects more conveniently.
* You get an alternative to inheritance when dealing with configuration presets for complex objects.

## Cons

* Cloning complex objects that have circular references might be very tricky.

## Relations with Other Patterns

* Many designs start by using [Factory Method](/design-patterns/factory-method) (less complicated and more customizable via subclasses) and evolve toward [Abstract Factory](/design-patterns/abstract-factory), [Prototype](/design-patterns/prototype), or [Builder](/design-patterns/builder) (more flexible, but more complicated).
* [Abstract Factory](/design-patterns/abstract-factory) classes are often based on a set of [Factory Methods](/design-patterns/factory-method), but you can also use [Prototype](/design-patterns/prototype) to compose the methods on these classes.
* [Prototype](/design-patterns/prototype) can help when you need to save copies of [Commands](/design-patterns/command) into history.
* Designs that make heavy use of [Composite](/design-patterns/composite) and [Decorator](/design-patterns/decorator) can often benefit from using [Prototype](/design-patterns/prototype). Applying the pattern lets you clone complex structures instead of re-constructing them from scratch.
* [Prototype](/design-patterns/prototype) isn’t based on inheritance, so it doesn’t have its drawbacks. On the other hand, _Prototype_ requires a complicated initialization of the cloned object. [Factory Method](/design-patterns/factory-method) is based on inheritance but doesn’t require an initialization step.
* Sometimes [Prototype](/design-patterns/prototype) can be a simpler alternative to [Memento](/design-patterns/memento). This works if the object, the state of which you want to store in the history, is fairly straightforward and doesn’t have links to external resources, or the links are easy to re-establish.
* [Abstract Factories](/design-patterns/abstract-factory), [Builders](/design-patterns/builder) and [Prototypes](/design-patterns/prototype) can all be implemented as [Singletons](/design-patterns/singleton).
## Relations

**Related patterns**

- [Factory Method](/en/design-patterns/factory-method.md)
- [Abstract Factory](/en/design-patterns/abstract-factory.md)
- [Builder](/en/design-patterns/builder.md)
- [Command](/en/design-patterns/command.md)
- [Composite](/en/design-patterns/composite.md)
- [Decorator](/en/design-patterns/decorator.md)
- [Memento](/en/design-patterns/memento.md)
- [Singleton](/en/design-patterns/singleton.md)

## Code Examples

### java

```java
package refactoring_guru.prototype.example.shapes;

import java.util.Objects;

public abstract class Shape {
    public int x;
    public int y;
    public String color;

    public Shape() {
    }

    public Shape(Shape target) {
        if (target != null) {
            this.x = target.x;
            this.y = target.y;
            this.color = target.color;
        }
    }

    public abstract Shape clone();

    @Override
    public boolean equals(Object object2) {
        if (!(object2 instanceof Shape)) return false;
        Shape shape2 = (Shape) object2;
        return shape2.x == x && shape2.y == y && Objects.equals(shape2.color, color);
    }
}

package refactoring_guru.prototype.example.shapes;

public class Circle extends Shape {
    public int radius;

    public Circle() {
    }

    public Circle(Circle target) {
        super(target);
        if (target != null) {
            this.radius = target.radius;
        }
    }

    @Override
    public Shape clone() {
        return new Circle(this);
    }

    @Override
    public boolean equals(Object object2) {
        if (!(object2 instanceof Circle) || !super.equals(object2)) return false;
        Circle shape2 = (Circle) object2;
        return shape2.radius == radius;
    }
}

package refactoring_guru.prototype.example.shapes;

public class Rectangle extends Shape {
    public int width;
    public int height;

    public Rectangle() {
    }

    public Rectangle(Rectangle target) {
        super(target);
        if (target != null) {
            this.width = target.width;
            this.height = target.height;
        }
    }

    @Override
    public Shape clone() {
        return new Rectangle(this);
    }

    @Override
    public boolean equals(Object object2) {
        if (!(object2 instanceof Rectangle) || !super.equals(object2)) return false;
        Rectangle shape2 = (Rectangle) object2;
        return shape2.width == width && shape2.height == height;
    }
}

package refactoring_guru.prototype.example;

import refactoring_guru.prototype.example.shapes.Circle;
import refactoring_guru.prototype.example.shapes.Rectangle;
import refactoring_guru.prototype.example.shapes.Shape;

import java.util.ArrayList;
import java.util.List;

public class Demo {
    public static void main(String[] args) {
        List<Shape> shapes = new ArrayList<>();
        List<Shape> shapesCopy = new ArrayList<>();

        Circle circle = new Circle();
        circle.x = 10;
        circle.y = 20;
        circle.radius = 15;
        circle.color = "red";
        shapes.add(circle);

        Circle anotherCircle = (Circle) circle.clone();
        shapes.add(anotherCircle);

        Rectangle rectangle = new Rectangle();
        rectangle.width = 10;
        rectangle.height = 20;
        rectangle.color = "blue";
        shapes.add(rectangle);

        cloneAndCompare(shapes, shapesCopy);
    }

    private static void cloneAndCompare(List<Shape> shapes, List<Shape> shapesCopy) {
        for (Shape shape : shapes) {
            shapesCopy.add(shape.clone());
        }

        for (int i = 0; i < shapes.size(); i++) {
            if (shapes.get(i) != shapesCopy.get(i)) {
                System.out.println(i + ": Shapes are different objects (yay!)");
                if (shapes.get(i).equals(shapesCopy.get(i))) {
                    System.out.println(i + ": And they are identical (yay!)");
                } else {
                    System.out.println(i + ": But they are not identical (booo!)");
                }
            } else {
                System.out.println(i + ": Shape objects are the same (booo!)");
            }
        }
    }
}

0: Shapes are different objects (yay!)
0: And they are identical (yay!)
1: Shapes are different objects (yay!)
1: And they are identical (yay!)
2: Shapes are different objects (yay!)
2: And they are identical (yay!)

package refactoring_guru.prototype.caching.cache;

import refactoring_guru.prototype.example.shapes.Circle;
import refactoring_guru.prototype.example.shapes.Rectangle;
import refactoring_guru.prototype.example.shapes.Shape;

import java.util.HashMap;
import java.util.Map;

public class BundledShapeCache {
    private Map<String, Shape> cache = new HashMap<>();

    public BundledShapeCache() {
        Circle circle = new Circle();
        circle.x = 5;
        circle.y = 7;
        circle.radius = 45;
        circle.color = "Green";

        Rectangle rectangle = new Rectangle();
        rectangle.x = 6;
        rectangle.y = 9;
        rectangle.width = 8;
        rectangle.height = 10;
        rectangle.color = "Blue";

        cache.put("Big green circle", circle);
        cache.put("Medium blue rectangle", rectangle);
    }

    public Shape put(String key, Shape shape) {
        cache.put(key, shape);
        return shape;
    }

    public Shape get(String key) {
        return cache.get(key).clone();
    }
}

package refactoring_guru.prototype.caching;

import refactoring_guru.prototype.caching.cache.BundledShapeCache;
import refactoring_guru.prototype.example.shapes.Shape;

public class Demo {
    public static void main(String[] args) {
        BundledShapeCache cache = new BundledShapeCache();

        Shape shape1 = cache.get("Big green circle");
        Shape shape2 = cache.get("Medium blue rectangle");
        Shape shape3 = cache.get("Medium blue rectangle");

        if (shape1 != shape2 && !shape1.equals(shape2)) {
            System.out.println("Big green circle != Medium blue rectangle (yay!)");
        } else {
            System.out.println("Big green circle == Medium blue rectangle (booo!)");
        }

        if (shape2 != shape3) {
            System.out.println("Medium blue rectangles are two different objects (yay!)");
            if (shape2.equals(shape3)) {
                System.out.println("And they are identical (yay!)");
            } else {
                System.out.println("But they are not identical (booo!)");
            }
        } else {
            System.out.println("Rectangle objects are the same (booo!)");
        }
    }
}

Big green circle != Medium blue rectangle (yay!)
Medium blue rectangles are two different objects (yay!)
And they are identical (yay!)
```

### csharp

```csharp
using System;

namespace RefactoringGuru.DesignPatterns.Prototype.Conceptual
{
    public class Person
    {
        public int Age;
        public DateTime BirthDate;
        public string Name;
        public IdInfo IdInfo;

        public Person ShallowCopy()
        {
            return (Person) this.MemberwiseClone();
        }

        public Person DeepCopy()
        {
            Person clone = (Person) this.MemberwiseClone();
            clone.IdInfo = new IdInfo(IdInfo.IdNumber);
            clone.Name = String.Copy(Name);
            return clone;
        }
    }

    public class IdInfo
    {
        public int IdNumber;

        public IdInfo(int idNumber)
        {
            this.IdNumber = idNumber;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Person p1 = new Person();
            p1.Age = 42;
            p1.BirthDate = Convert.ToDateTime("1977-01-01");
            p1.Name = "Jack Daniels";
            p1.IdInfo = new IdInfo(666);

            // Perform a shallow copy of p1 and assign it to p2.
            Person p2 = p1.ShallowCopy();
            // Make a deep copy of p1 and assign it to p3.
            Person p3 = p1.DeepCopy();

            // Display values of p1, p2 and p3.
            Console.WriteLine("Original values of p1, p2, p3:");
            Console.WriteLine("   p1 instance values: ");
            DisplayValues(p1);
            Console.WriteLine("   p2 instance values:");
            DisplayValues(p2);
            Console.WriteLine("   p3 instance values:");
            DisplayValues(p3);

            // Change the value of p1 properties and display the values of p1,
            // p2 and p3.
            p1.Age = 32;
            p1.BirthDate = Convert.ToDateTime("1900-01-01");
            p1.Name = "Frank";
            p1.IdInfo.IdNumber = 7878;
            Console.WriteLine("\nValues of p1, p2 and p3 after changes to p1:");
            Console.WriteLine("   p1 instance values: ");
            DisplayValues(p1);
            Console.WriteLine("   p2 instance values (reference values have changed):");
            DisplayValues(p2);
            Console.WriteLine("   p3 instance values (everything was kept the same):");
            DisplayValues(p3);
        }

        public static void DisplayValues(Person p)
        {
            Console.WriteLine("      Name: {0:s}, Age: {1:d}, BirthDate: {2:MM/dd/yy}",
                p.Name, p.Age, p.BirthDate);
            Console.WriteLine("      ID#: {0:d}", p.IdInfo.IdNumber);
        }
    }
}

Original values of p1, p2, p3:
   p1 instance values: 
      Name: Jack Daniels, Age: 42, BirthDate: 01/01/77
      ID#: 666
   p2 instance values:
      Name: Jack Daniels, Age: 42, BirthDate: 01/01/77
      ID#: 666
   p3 instance values:
      Name: Jack Daniels, Age: 42, BirthDate: 01/01/77
      ID#: 666

Values of p1, p2 and p3 after changes to p1:
   p1 instance values: 
      Name: Frank, Age: 32, BirthDate: 01/01/00
      ID#: 7878
   p2 instance values (reference values have changed):
      Name: Jack Daniels, Age: 42, BirthDate: 01/01/77
      ID#: 7878
   p3 instance values (everything was kept the same):
      Name: Jack Daniels, Age: 42, BirthDate: 01/01/77
      ID#: 666
```

### cpp

```cpp
using std::string;

// Prototype Design Pattern
//
// Intent: Lets you copy existing objects without making your code dependent on
// their classes.

enum Type {
  PROTOTYPE_1 = 0,
  PROTOTYPE_2
};

/**
 * The example class that has cloning ability. We'll see how the values of field
 * with different types will be cloned.
 */

class Prototype {
 protected:
  string prototype_name_;
  float prototype_field_;

 public:
  Prototype() {}
  Prototype(string prototype_name)
      : prototype_name_(prototype_name) {
  }
  virtual ~Prototype() {}
  virtual Prototype *Clone() const = 0;
  virtual void Method(float prototype_field) {
    this->prototype_field_ = prototype_field;
    std::cout << "Call Method from " << prototype_name_ << " with field : " << prototype_field << std::endl;
  }
};

/**
 * ConcretePrototype1 is a Sub-Class of Prototype and implement the Clone Method
 * In this example all data members of Prototype Class are in the Stack. If you
 * have pointers in your properties for ex: String* name_ ,you will need to
 * implement the Copy-Constructor to make sure you have a deep copy from the
 * clone method
 */

class ConcretePrototype1 : public Prototype {
 private:
  float concrete_prototype_field1_;

 public:
  ConcretePrototype1(string prototype_name, float concrete_prototype_field)
      : Prototype(prototype_name), concrete_prototype_field1_(concrete_prototype_field) {
  }

  /**
   * Notice that Clone method return a Pointer to a new ConcretePrototype1
   * replica. so, the client (who call the clone method) has the responsability
   * to free that memory. If you have smart pointer knowledge you may prefer to
   * use unique_pointer here.
   */
  Prototype *Clone() const override {
    return new ConcretePrototype1(*this);
  }
};

class ConcretePrototype2 : public Prototype {
 private:
  float concrete_prototype_field2_;

 public:
  ConcretePrototype2(string prototype_name, float concrete_prototype_field)
      : Prototype(prototype_name), concrete_prototype_field2_(concrete_prototype_field) {
  }
  Prototype *Clone() const override {
    return new ConcretePrototype2(*this);
  }
};

/**
 * In PrototypeFactory you have two concrete prototypes, one for each concrete
 * prototype class, so each time you want to create a bullet , you can use the
 * existing ones and clone those.
 */

class PrototypeFactory {
 private:
  std::unordered_map<Type, Prototype *, std::hash<int>> prototypes_;

 public:
  PrototypeFactory() {
    prototypes_[Type::PROTOTYPE_1] = new ConcretePrototype1("PROTOTYPE_1 ", 50.f);
    prototypes_[Type::PROTOTYPE_2] = new ConcretePrototype2("PROTOTYPE_2 ", 60.f);
  }

  /**
   * Be carefull of free all memory allocated. Again, if you have smart pointers
   * knowelege will be better to use it here.
   */

  ~PrototypeFactory() {
    delete prototypes_[Type::PROTOTYPE_1];
    delete prototypes_[Type::PROTOTYPE_2];
  }

  /**
   * Notice here that you just need to specify the type of the prototype you
   * want and the method will create from the object with this type.
   */
  Prototype *CreatePrototype(Type type) {
    return prototypes_[type]->Clone();
  }
};

void Client(PrototypeFactory &prototype_factory) {
  std::cout << "Let's create a Prototype 1\n";

  Prototype *prototype = prototype_factory.CreatePrototype(Type::PROTOTYPE_1);
  prototype->Method(90);
  delete prototype;

  std::cout << "\n";

  std::cout << "Let's create a Prototype 2 \n";

  prototype = prototype_factory.CreatePrototype(Type::PROTOTYPE_2);
  prototype->Method(10);

  delete prototype;
}

int main() {
  PrototypeFactory *prototype_factory = new PrototypeFactory();
  Client(*prototype_factory);
  delete prototype_factory;

  return 0;
}

Let's create a Prototype 1
Call Method from PROTOTYPE_1  with field : 90

Let's create a Prototype 2 
Call Method from PROTOTYPE_2  with field : 10
```

### go

```go
package main

type Inode interface {
	print(string)
	clone() Inode
}

package main

import "fmt"

type File struct {
	name string
}

func (f *File) print(indentation string) {
	fmt.Println(indentation + f.name)
}

func (f *File) clone() Inode {
	return &File{name: f.name + "_clone"}
}

package main

import "fmt"

type Folder struct {
	children []Inode
	name     string
}

func (f *Folder) print(indentation string) {
	fmt.Println(indentation + f.name)
	for _, i := range f.children {
		i.print(indentation + indentation)
	}
}

func (f *Folder) clone() Inode {
	cloneFolder := &Folder{name: f.name + "_clone"}
	var tempChildren []Inode
	for _, i := range f.children {
		copy := i.clone()
		tempChildren = append(tempChildren, copy)
	}
	cloneFolder.children = tempChildren
	return cloneFolder
}

package main

import "fmt"

func main() {
	file1 := &File{name: "File1"}
	file2 := &File{name: "File2"}
	file3 := &File{name: "File3"}

	folder1 := &Folder{
		children: []Inode{file1},
		name:     "Folder1",
	}

	folder2 := &Folder{
		children: []Inode{folder1, file2, file3},
		name:     "Folder2",
	}
	fmt.Println("\nPrinting hierarchy for Folder2")
	folder2.print("  ")

	cloneFolder := folder2.clone()
	fmt.Println("\nPrinting hierarchy for clone Folder")
	cloneFolder.print("  ")
}

Printing hierarchy for Folder2
  Folder2
    Folder1
        File1
    File2
    File3

Printing hierarchy for clone Folder
  Folder2_clone
    Folder1_clone
        File1_clone
    File2_clone
    File3_clone
```

### php

```php
<?php

namespace RefactoringGuru\Prototype\Conceptual;

/**
 * The example class that has cloning ability. We'll see how the values of field
 * with different types will be cloned.
 */
class Prototype
{
    public $primitive;
    public $component;
    public $circularReference;

    /**
     * PHP has built-in cloning support. You can `clone` an object without
     * defining any special methods as long as it has fields of primitive types.
     * Fields containing objects retain their references in a cloned object.
     * Therefore, in some cases, you might want to clone those referenced
     * objects as well. You can do this in a special `__clone()` method.
     */
    public function __clone()
    {
        $this->component = clone $this->component;

        // Cloning an object that has a nested object with backreference
        // requires special treatment. After the cloning is completed, the
        // nested object should point to the cloned object, instead of the
        // original object.
        $this->circularReference = clone $this->circularReference;
        $this->circularReference->prototype = $this;
    }
}

class ComponentWithBackReference
{
    public $prototype;

    /**
     * Note that the constructor won't be executed during cloning. If you have
     * complex logic inside the constructor, you may need to execute it in the
     * `__clone` method as well.
     */
    public function __construct(Prototype $prototype)
    {
        $this->prototype = $prototype;
    }
}

/**
 * The client code.
 */
function clientCode()
{
    $p1 = new Prototype();
    $p1->primitive = 245;
    $p1->component = new \DateTime();
    $p1->circularReference = new ComponentWithBackReference($p1);

    $p2 = clone $p1;
    if ($p1->primitive === $p2->primitive) {
        echo "Primitive field values have been carried over to a clone. Yay!\n";
    } else {
        echo "Primitive field values have not been copied. Booo!\n";
    }
    if ($p1->component === $p2->component) {
        echo "Simple component has not been cloned. Booo!\n";
    } else {
        echo "Simple component has been cloned. Yay!\n";
    }

    if ($p1->circularReference === $p2->circularReference) {
        echo "Component with back reference has not been cloned. Booo!\n";
    } else {
        echo "Component with back reference has been cloned. Yay!\n";
    }

    if ($p1->circularReference->prototype === $p2->circularReference->prototype) {
        echo "Component with back reference is linked to original object. Booo!\n";
    } else {
        echo "Component with back reference is linked to the clone. Yay!\n";
    }
}

clientCode();

Primitive field values have been carried over to a clone. Yay!
Simple component has been cloned. Yay!
Component with back reference has been cloned. Yay!
Component with back reference is linked to the clone. Yay!

<?php

namespace RefactoringGuru\Prototype\RealWorld;

/**
 * Prototype.
 */
class Page
{
    private $title;

    private $body;

    /**
     * @var Author
     */
    private $author;

    private $comments = [];

    /**
     * @var \DateTime
     */
    private $date;

    // +100 private fields.

    public function __construct(string $title, string $body, Author $author)
    {
        $this->title = $title;
        $this->body = $body;
        $this->author = $author;
        $this->author->addToPage($this);
        $this->date = new \DateTime();
    }

    public function addComment(string $comment): void
    {
        $this->comments[] = $comment;
    }

    /**
     * You can control what data you want to carry over to the cloned object.
     *
     * For instance, when a page is cloned:
     * - It gets a new "Copy of ..." title.
     * - The author of the page remains the same. Therefore we leave the
     * reference to the existing object while adding the cloned page to the list
     * of the author's pages.
     * - We don't carry over the comments from the old page.
     * - We also attach a new date object to the page.
     */
    public function __clone()
    {
        $this->title = "Copy of " . $this->title;
        $this->author->addToPage($this);
        $this->comments = [];
        $this->date = new \DateTime();
    }
}

class Author
{
    private $name;

    /**
     * @var Page[]
     */
    private $pages = [];

    public function __construct(string $name)
    {
        $this->name = $name;
    }

    public function addToPage(Page $page): void
    {
        $this->pages[] = $page;
    }
}

/**
 * The client code.
 */
function clientCode()
{
    $author = new Author("John Smith");
    $page = new Page("Tip of the day", "Keep calm and carry on.", $author);

    // ...

    $page->addComment("Nice tip, thanks!");

    // ...

    $draft = clone $page;
    echo "Dump of the clone. Note that the author is now referencing two objects.\n\n";
    print_r($draft);
}

clientCode();

Dump of the clone. Note that the author is now referencing two objects.

RefactoringGuru\Prototype\RealWorld\Page Object
(
    [title:RefactoringGuru\Prototype\RealWorld\Page:private] => Copy of Tip of the day
    [body:RefactoringGuru\Prototype\RealWorld\Page:private] => Keep calm and carry on.
    [author:RefactoringGuru\Prototype\RealWorld\Page:private] => RefactoringGuru\Prototype\RealWorld\Author Object
        (
            [name:RefactoringGuru\Prototype\RealWorld\Author:private] => John Smith
            [pages:RefactoringGuru\Prototype\RealWorld\Author:private] => Array
                (
                    [0] => RefactoringGuru\Prototype\RealWorld\Page Object
                        (
                            [title:RefactoringGuru\Prototype\RealWorld\Page:private] => Tip of the day
                            [body:RefactoringGuru\Prototype\RealWorld\Page:private] => Keep calm and carry on.
                            [author:RefactoringGuru\Prototype\RealWorld\Page:private] => RefactoringGuru\Prototype\RealWorld\Author Object
 *RECURSION*
                            [comments:RefactoringGuru\Prototype\RealWorld\Page:private] => Array
                                (
                                    [0] => Nice tip, thanks!
                                )

                            [date:RefactoringGuru\Prototype\RealWorld\Page:private] => DateTime Object
                                (
                                    [date] => 2018-06-04 14:50:39.306237
                                    [timezone_type] => 3
                                    [timezone] => UTC
                                )

                        )

                    [1] => RefactoringGuru\Prototype\RealWorld\Page Object
 *RECURSION*
                )

        )

    [comments:RefactoringGuru\Prototype\RealWorld\Page:private] => Array
        (
        )

    [date:RefactoringGuru\Prototype\RealWorld\Page:private] => DateTime Object
        (
            [date] => 2018-06-04 14:50:39.306272
            [timezone_type] => 3
            [timezone] => UTC
        )

)
```

### python

```python
import copy


class SelfReferencingEntity:
    def __init__(self):
        self.parent = None

    def set_parent(self, parent):
        self.parent = parent


class SomeComponent:
    """
    Python provides its own interface of Prototype via `copy.copy` and
    `copy.deepcopy` functions. And any class that wants to implement custom
    implementations have to override `__copy__` and `__deepcopy__` member
    functions.
    """

    def __init__(self, some_int, some_list_of_objects, some_circular_ref):
        self.some_int = some_int
        self.some_list_of_objects = some_list_of_objects
        self.some_circular_ref = some_circular_ref

    def __copy__(self):
        """
        Create a shallow copy. This method will be called whenever someone calls
        `copy.copy` with this object and the returned value is returned as the
        new shallow copy.
        """

        # First, let's create copies of the nested objects.
        some_list_of_objects = copy.copy(self.some_list_of_objects)
        some_circular_ref = copy.copy(self.some_circular_ref)

        # Then, let's clone the object itself, using the prepared clones of the
        # nested objects.
        new = self.__class__(
            self.some_int, some_list_of_objects, some_circular_ref
        )
        new.__dict__.update(self.__dict__)

        return new

    def __deepcopy__(self, memo=None):
        """
        Create a deep copy. This method will be called whenever someone calls
        `copy.deepcopy` with this object and the returned value is returned as
        the new deep copy.

        What is the use of the argument `memo`? Memo is the dictionary that is
        used by the `deepcopy` library to prevent infinite recursive copies in
        instances of circular references. Pass it to all the `deepcopy` calls
        you make in the `__deepcopy__` implementation to prevent infinite
        recursions.
        """
        if memo is None:
            memo = {}

        # First, let's create copies of the nested objects.
        some_list_of_objects = copy.deepcopy(self.some_list_of_objects, memo)
        some_circular_ref = copy.deepcopy(self.some_circular_ref, memo)

        # Then, let's clone the object itself, using the prepared clones of the
        # nested objects.
        new = self.__class__(
            self.some_int, some_list_of_objects, some_circular_ref
        )
        new.__dict__ = copy.deepcopy(self.__dict__, memo)

        return new


if __name__ == "__main__":

    list_of_objects = [1, {1, 2, 3}, [1, 2, 3]]
    circular_ref = SelfReferencingEntity()
    component = SomeComponent(23, list_of_objects, circular_ref)
    circular_ref.set_parent(component)

    shallow_copied_component = copy.copy(component)

    # Let's change the list in shallow_copied_component and see if it changes in
    # component.
    shallow_copied_component.some_list_of_objects.append("another object")
    if component.some_list_of_objects[-1] == "another object":
        print(
            "Adding elements to `shallow_copied_component`'s "
            "some_list_of_objects adds it to `component`'s "
            "some_list_of_objects."
        )
    else:
        print(
            "Adding elements to `shallow_copied_component`'s "
            "some_list_of_objects doesn't add it to `component`'s "
            "some_list_of_objects."
        )

    # Let's change the set in the list of objects.
    component.some_list_of_objects[1].add(4)
    if 4 in shallow_copied_component.some_list_of_objects[1]:
        print(
            "Changing objects in the `component`'s some_list_of_objects "
            "changes that object in `shallow_copied_component`'s "
            "some_list_of_objects."
        )
    else:
        print(
            "Changing objects in the `component`'s some_list_of_objects "
            "doesn't change that object in `shallow_copied_component`'s "
            "some_list_of_objects."
        )

    deep_copied_component = copy.deepcopy(component)

    # Let's change the list in deep_copied_component and see if it changes in
    # component.
    deep_copied_component.some_list_of_objects.append("one more object")
    if component.some_list_of_objects[-1] == "one more object":
        print(
            "Adding elements to `deep_copied_component`'s "
            "some_list_of_objects adds it to `component`'s "
            "some_list_of_objects."
        )
    else:
        print(
            "Adding elements to `deep_copied_component`'s "
            "some_list_of_objects doesn't add it to `component`'s "
            "some_list_of_objects."
        )

    # Let's change the set in the list of objects.
    component.some_list_of_objects[1].add(10)
    if 10 in deep_copied_component.some_list_of_objects[1]:
        print(
            "Changing objects in the `component`'s some_list_of_objects "
            "changes that object in `deep_copied_component`'s "
            "some_list_of_objects."
        )
    else:
        print(
            "Changing objects in the `component`'s some_list_of_objects "
            "doesn't change that object in `deep_copied_component`'s "
            "some_list_of_objects."
        )

    print(
        f"id(deep_copied_component.some_circular_ref.parent): "
        f"{id(deep_copied_component.some_circular_ref.parent)}"
    )
    print(
        f"id(deep_copied_component.some_circular_ref.parent.some_circular_ref.parent): "
        f"{id(deep_copied_component.some_circular_ref.parent.some_circular_ref.parent)}"
    )
    print(
        "^^ This shows that deepcopied objects contain same reference, they "
        "are not cloned repeatedly."
    )

Adding elements to `shallow_copied_component`'s some_list_of_objects adds it to `component`'s some_list_of_objects.
Changing objects in the `component`'s some_list_of_objects changes that object in `shallow_copied_component`'s some_list_of_objects.
Adding elements to `deep_copied_component`'s some_list_of_objects doesn't add it to `component`'s some_list_of_objects.
Changing objects in the `component`'s some_list_of_objects doesn't change that object in `deep_copied_component`'s some_list_of_objects.
id(deep_copied_component.some_circular_ref.parent): 4429472784
id(deep_copied_component.some_circular_ref.parent.some_circular_ref.parent): 4429472784
^^ This shows that deepcopied objects contain same reference, they are not cloned repeatedly.
```

### ruby

```ruby
# The example class that has cloning ability. We'll see how the values of field
# with different types will be cloned.
class Prototype
  attr_accessor :primitive, :component, :circular_reference

  def initialize
    @primitive = nil
    @component = nil
    @circular_reference = nil
  end

  # @return [Prototype]
  def clone
    @component = deep_copy(@component)

    # Cloning an object that has a nested object with backreference requires
    # special treatment. After the cloning is completed, the nested object
    # should point to the cloned object, instead of the original object.
    @circular_reference = deep_copy(@circular_reference)
    @circular_reference.prototype = self
    deep_copy(self)
  end

  # deep_copy is the usual Marshalling hack to make a deep copy. But it's rather
  # slow and inefficient, therefore, in real applications, use a special gem.
  private def deep_copy(object)
    Marshal.load(Marshal.dump(object))
  end
end

class ComponentWithBackReference
  attr_accessor :prototype

  # @param [Prototype] prototype
  def initialize(prototype)
    @prototype = prototype
  end
end

# The client code.
p1 = Prototype.new
p1.primitive = 245
p1.component = Time.now
p1.circular_reference = ComponentWithBackReference.new(p1)

p2 = p1.clone

if p1.primitive == p2.primitive
  puts 'Primitive field values have been carried over to a clone. Yay!'
else
  puts 'Primitive field values have not been copied. Booo!'
end

if p1.component.equal?(p2.component)
  puts 'Simple component has not been cloned. Booo!'
else
  puts 'Simple component has been cloned. Yay!'
end

if p1.circular_reference.equal?(p2.circular_reference)
  puts 'Component with back reference has not been cloned. Booo!'
else
  puts 'Component with back reference has been cloned. Yay!'
end

if p1.circular_reference.prototype.equal?(p2.circular_reference.prototype)
  print 'Component with back reference is linked to original object. Booo!'
else
  print 'Component with back reference is linked to the clone. Yay!'
end

Primitive field values have been carried over to a clone. Yay!
Simple component has been cloned. Yay!
Component with back reference has been cloned. Yay!
Component with back reference is linked to the clone. Yay!
```

### rust

```rust
#[derive(Clone)]
struct Circle {
    pub x: u32,
    pub y: u32,
    pub radius: u32,
}

fn main() {
    let circle1 = Circle {
        x: 10,
        y: 15,
        radius: 10,
    };

    // Prototype in action.
    let mut circle2 = circle1.clone();
    circle2.radius = 77;

    println!("Circle 1: {}, {}, {}", circle1.x, circle1.y, circle1.radius);
    println!("Circle 2: {}, {}, {}", circle2.x, circle2.y, circle2.radius);
}

Circle 1: 10, 15, 10
Circle 2: 10, 15, 77
```

### swift

```swift
import XCTest

/// Swift has built-in cloning support. To add cloning support to your class,
/// you need to implement the NSCopying protocol in that class and provide the
/// implementation for the `copy` method.
class BaseClass: NSCopying, Equatable {

    private var intValue = 1
    private var stringValue = "Value"

    required init(intValue: Int = 1, stringValue: String = "Value") {

        self.intValue = intValue
        self.stringValue = stringValue
    }

    /// MARK: - NSCopying
    func copy(with zone: NSZone? = nil) -> Any {
        let prototype = type(of: self).init()
        prototype.intValue = intValue
        prototype.stringValue = stringValue
        print("Values defined in BaseClass have been cloned!")
        return prototype
    }

    /// MARK: - Equatable
    static func == (lhs: BaseClass, rhs: BaseClass) -> Bool {
        return lhs.intValue == rhs.intValue && lhs.stringValue == rhs.stringValue
    }
}

/// Subclasses can override the base `copy` method to copy their own data into
/// the resulting object. But you should always call the base method first.
class SubClass: BaseClass {

    private var boolValue = true

    func copy() -> Any {
        return copy(with: nil)
    }

    override func copy(with zone: NSZone?) -> Any {
        guard let prototype = super.copy(with: zone) as? SubClass else {
            return SubClass() // oops
        }
        prototype.boolValue = boolValue
        print("Values defined in SubClass have been cloned!")
        return prototype
    }
}

/// The client code.
class Client {
    // ...
    static func someClientCode() {
        let original = SubClass(intValue: 2, stringValue: "Value2")

        guard let copy = original.copy() as? SubClass else {
            XCTAssert(false)
            return
        }

        /// See implementation of `Equatable` protocol for more details.
        XCTAssert(copy == original)

        print("The original object is equal to the copied object!")
    }
    // ...
}

/// Let's see how it all works together.
class PrototypeConceptual: XCTestCase {

    func testPrototype_NSCopying() {
        Client.someClientCode()
    }
}

Values defined in BaseClass have been cloned!
Values defined in SubClass have been cloned!
The original object is equal to the copied object!

import XCTest

class PrototypeRealWorld: XCTestCase {

    func testPrototypeRealWorld() {

        let author = Author(id: 10, username: "Ivan_83")
        let page = Page(title: "My First Page", contents: "Hello world!", author: author)

        page.add(comment: Comment(message: "Keep it up!"))

        /// Since NSCopying returns Any, the copied object should be unwrapped.
        guard let anotherPage = page.copy() as? Page else {
            XCTFail("Page was not copied")
            return
        }

        /// Comments should be empty as it is a new page.
        XCTAssert(anotherPage.comments.isEmpty)

        /// Note that the author is now referencing two objects.
        XCTAssert(author.pagesCount == 2)

        print("Original title: " + page.title)
        print("Copied title: " + anotherPage.title)
        print("Count of pages: " + String(author.pagesCount))
    }
}

private class Author {

    private var id: Int
    private var username: String
    private var pages = [Page]()

    init(id: Int, username: String) {
        self.id = id
        self.username = username
    }

    func add(page: Page) {
        pages.append(page)
    }

    var pagesCount: Int {
        return pages.count
    }
}

private class Page: NSCopying {

    private(set) var title: String
    private(set) var contents: String
    private weak var author: Author?
    private(set) var comments = [Comment]()

    init(title: String, contents: String, author: Author?) {
        self.title = title
        self.contents = contents
        self.author = author
        author?.add(page: self)
    }

    func add(comment: Comment) {
        comments.append(comment)
    }

    /// MARK: - NSCopying

    func copy(with zone: NSZone? = nil) -> Any {
        return Page(title: "Copy of '" + title + "'", contents: contents, author: author)
    }
}

private struct Comment {

    let date = Date()
    let message: String
}

Original title: My First Page
Copied title: Copy of 'My First Page'
Count of pages: 2
```

### typescript

```typescript
/**
 * The example class that has cloning ability. We'll see how the values of field
 * with different types will be cloned.
 */
class Prototype {
    public primitive: any;
    public component: object;
    public circularReference: ComponentWithBackReference;

    public clone(): this {
        const clone = Object.create(this);

        clone.component = Object.create(this.component);

        // Cloning an object that has a nested object with backreference
        // requires special treatment. After the cloning is completed, the
        // nested object should point to the cloned object, instead of the
        // original object. Spread operator can be handy for this case.
        clone.circularReference = new ComponentWithBackReference(clone);

        return clone;
    }
}

class ComponentWithBackReference {
    public prototype;

    constructor(prototype: Prototype) {
        this.prototype = prototype;
    }
}

/**
 * The client code.
 */
function clientCode() {
    const p1 = new Prototype();
    p1.primitive = 245;
    p1.component = new Date();
    p1.circularReference = new ComponentWithBackReference(p1);

    const p2 = p1.clone();
    if (p1.primitive === p2.primitive) {
        console.log('Primitive field values have been carried over to a clone. Yay!');
    } else {
        console.log('Primitive field values have not been copied. Booo!');
    }
    if (p1.component === p2.component) {
        console.log('Simple component has not been cloned. Booo!');
    } else {
        console.log('Simple component has been cloned. Yay!');
    }

    if (p1.circularReference === p2.circularReference) {
        console.log('Component with back reference has not been cloned. Booo!');
    } else {
        console.log('Component with back reference has been cloned. Yay!');
    }

    if (p1.circularReference.prototype === p2.circularReference.prototype) {
        console.log('Component with back reference is linked to original object. Booo!');
    } else {
        console.log('Component with back reference is linked to the clone. Yay!');
    }
}

clientCode();

Primitive field values have been carried over to a clone. Yay!
Simple component has been cloned. Yay!
Component with back reference has been cloned. Yay!
Component with back reference is linked to the clone. Yay!
```