惰性初始模式

在程式設計中, 惰性初始是一種拖延戰術。在第一次需求出現以前，先延遲創建物件、計算值或其它昂貴程序。

這通常是以一個旗號來實現，用旗號來標示是否完成其程式。每次請求對象時，會先測試此旗號。如果已完成，直接傳回，否則當場執行。

對於此想法更一般的論述，可見惰性求值。

對指令式語言，這個模式可能潛藏著危險，尤其是使用共享狀態的程式習慣。

"惰性工廠" 
以設計模式的觀點來說，惰性初始通常會和工廠方法模式合作，這結合了三種構想：
 使用一個工廠去得到一個類別的實例（工廠方法模式）。
 將實例存在一個集合中，所以下次要求一個實例卻有相同參數時，可以得到同一個實例（可和單例模式來做比較）。
 在第一次時，使用惰性初始來實例化物件（惰性初始模式）。

示例

C# 
Fruit 類別本身在這裡不做任合事。_typesDictionary 變數則是一個存 Fruit 實例的 Dictionary 或 Map ，其透過typeName來存取。

using System;
using System.Collections;
using System.Collections.Generic;

public class Fruit
{
    private string _typeName;

    private static Dictionary<string, Fruit> _typesDictionary = new Dictionary<string, Fruit>();

    private Fruit(String typeName)
    {
        this._typeName = typeName;
    }

    public static Fruit GetFruitByTypeName(string type)
    {
        Fruit fruit;

        if (!_typesDictionary.ContainsKey(type))
        {
            // 惰性初始
            fruit = new Fruit(type);

            _typesDictionary.Add(type, fruit);
        }
        else
            fruit = _typesDictionary[type];

        return fruit;
    }

    public static void ShowAll()
    {
        if (_typesDictionary.Count > 0)
        {
            Console.WriteLine("Number of instances made = {0}", _typesDictionary.Count);
            
            foreach (KeyValuePair<string, Fruit> kvp in _typesDictionary)
            {
                Console.WriteLine(kvp.Key);
            }
            
            Console.WriteLine();
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        Fruit.GetFruitByTypeName("Banana");
        Fruit.ShowAll();

        Fruit.GetFruitByTypeName("Apple");
        Fruit.ShowAll();

        // returns pre-existing instance from first 
        // time Fruit with "Banana" was created
        Fruit.GetFruitByTypeName("Banana");
        Fruit.ShowAll();

        Console.ReadLine();
    }
}

Java 
import java.util.*;
  
public class Fruit
{
    private static final Map<String,Fruit> types = new HashMap<String,Fruit>();
    private final String type;
  
    // using a private constructor to force use of the factory method.
    private Fruit(String type) {
      this.type = type;
    }
    
    /**
     * Lazy Factory method, gets the Fruit instance associated with a
     * certain type. Instantiates new ones as needed.
     * @param type Any string that describes a fruit type, e.g. "apple"
     * @return The Fruit instance associated with that type.
     */
    public static synchronized Fruit getFruit(String type) {
      if(!types.containsKey(type))
        types.put(type, new Fruit(type)); // Lazy initialization
      return types.get(type);
    }
}

C++ 
#include <iostream>
#include <string>
#include <map>
 
using namespace std;
 
class Fruit {
    public:
        static Fruit* getFruit(const string& type);
        static void printCurrentTypes();

    private:
        static map<string,Fruit*> types;
        string type;

        // note: constructor private forcing one to use static getFruit()
        Fruit(const string& t) : type( t ) {}
};

//definition needed for using any static member variable
map<string,Fruit*> Fruit::types; 
 
/*
 * Lazy Factory method, gets the Fruit instance associated with a
 * certain type. Instantiates new ones as needed.
 * precondition: type. Any string that describes a fruit type, e.g. "apple"
 * postcondition: The Fruit instance associated with that type.
 */
Fruit* Fruit::getFruit(const string& type) {
    Fruit *& f = types[type];   // try to find a pre-existing instance, or std::map'll create one if not found
 
    if (!f) {                   // if it was created by map automatically, it'll be pointing to NULL
        // couldn't find one, so make a new instance
        f = new Fruit(type); // lazy initialization part
    }
    return f;
}

/*
 * For example purposes to see pattern in action
 */
void Fruit::printCurrentTypes() {
    if (!types.empty()) {
        cout << "Number of instances made = " << types.size() << endl;
        for (map<string,Fruit*>::iterator iter = types.begin(); iter != types.end(); ++iter) {
            cout << (*iter).first << endl;
        }
        cout << endl;
    }
}

int main(void) {
    Fruit::getFruit("Banana");
    Fruit::printCurrentTypes();
 
    Fruit::getFruit("Apple");
    Fruit::printCurrentTypes();
 
    // returns pre-existing instance from first 
    // time Fruit with "Banana" was created
    Fruit::getFruit("Banana");
    Fruit::printCurrentTypes();
 
    return 0;
}

/*
輸出：
Number of instances made = 1
Banana
 
Number of instances made = 2
Apple
Banana
 
Number of instances made = 2
Apple
Banana
*/

Smalltalk 
The following is an example (in Smalltalk) of a typical accessor method to return the value of a variable using lazy initialization. 

    height
        height ifNil: [height := 2.0].
        ^height
</source>
The 'non-lazy' alternative is to use an initialization method that is run when the object is created and then use a simpler accessor method to fetch the value.

<source lang="smalltalk">
    initialize
        height := 2.0

    height
        ^height

Note that lazy initialization can also be used in non-object-oriented languages.

Ruby 
The following is an example (in Ruby) of lazily initializing an authentication token from a remote service like Google. The way that @auth_token is cached is also an example of memoization.

require 'net/http'
class Blogger
  def auth_token
    @auth_token ||=
      (res = Net::HTTP.post_form(uri, params)) &&
      get_token_from_http_response(res)
  end

  # get_token_from_http_response, uri and params are defined later in the class
end

b = Blogger.new
b.instance_variable_get(:@auth_token) # returns nil
b.auth_token # returns token
b.instance_variable_get(:@auth_token) # returns token

Python 

class Fruit:
    def __init__(self, type):
        self.type = type
    
class Fruits:
    def __init__(self):
        self.types = {}
    
    def get_fruit(self, type):
        if type not in self.types:
            self.types[type] = Fruit(type)
        
        return self.types[type]

if __name__ == '__main__':
    fruits = Fruits()
    print fruits.get_fruit('Apple')
    print fruits.get_fruit('Lime')

PHP 
<?php
header('Content-type:text/plain; charset=utf-8');

class Fruit
{
    private $type;
    private static $types = array();

    private function __construct($type)
    {
        $this->type = $type;
    }

    public static function getFruit($type)
    {
        // Lazy initialization takes place here
        if(!array_key_exists($type, self::$types)) {
            self::$types[$type] = new Fruit($type);
        }

        return self::$types[$type];
    }

    public static function printCurrentTypes()
    {
        echo 'Number of instances made: ' . count(self::$types) . "\n";

        foreach(array_keys(self::$types) as $key) echo "$key\n";

        echo "\n";
    }
}

Fruit::getFruit('Apple');
Fruit::printCurrentTypes();

Fruit::getFruit('Banana');
Fruit::printCurrentTypes();

Fruit::getFruit('Apple');
Fruit::printCurrentTypes();

/*
OUTPUT:

Number of instances made: 1
Apple

Number of instances made: 2
Apple
Banana

Number of instances made: 2
Apple
Banana
*/
?>

另見 
 代理模式
 單例模式
 Double-checked locking

外部連結 
 Article "Java Tip 67: Lazy instantiation - Balancing performance and resource usage" by Philip Bishop and Nigel Warren
 Java code examples 
 Use Lazy Initialization to Conserve Resources 
 Description from the Portland Pattern Repository 
 Lazy Initialization of Application Server Services
 Lazy Inheritance in JavaScript 

軟體設計模式