Prototype Pattern

Prototype is a creational design pattern that allows cloning objects, even complex ones, without coupling to their specific classes.

All prototype classes should have a common interface that makes it possible to copy objects even if their concrete classes are unknown. Prototype objects can produce full copies since objects of the same class can access each other’s private fields.

Golang Implementation

Example:

Let’s try to figure out the Prototype pattern using an example based on the operating system’s file system. The OS file system is recursive: the folders contain files and folders, which may also include files and folders, and so on.

Each file and folder can be represented by an inode interface. inode interface also has the clone function.

Both file and folder structs implement the print and clone functions since they are of the inode type. Also, notice the clone function in both file and folder. The clone function in both of them returns a copy of the respective file or folder. During the cloning, we append the suffix “_clone” to the name field.

inode.go

package main

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

file.go

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"}
}

folder.go

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
}

main.go

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

Rust Implementation

Example:

Rust has a built-in std::clone::Clone trait with many implementations for various types (via #[derive(Clone)]). Thus, the Prototype pattern is ready to use out of the box.

main.rs

#[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