Lazy initialization

Updated on Nov 25, 2023

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In computer programming, lazy initialization is the tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed. It is a kind of lazy evaluation that refers specifically to the initialization of objects or other resources.

This is typically accomplished by augmenting an accessor method (or property getter) to check whether a private member, acting as a cache, has already been initialized. If it has, it is returned straight away. If not, a new instance is created, placed into the member variable, and returned to the caller just-in-time for its first use.

If objects have properties that are rarely used, this can improve startup speed. Mean average program performance may be slightly worse in terms of memory (for the condition variables) and and execution cycles (to check them), but the impact of object instantiation is spread in time ("amortized") rather than concentrated in the startup phase of a system, and thus median response times can be greatly improved.

In multithreaded code, access to lazy-initialized objects/state must be synchronized to guard against race conditions.

The "lazy factory"

In a software design pattern view, lazy initialization is often used together with a factory method pattern. This combines three ideas:

Using a factory method to create instances of a class (factory method pattern)

Storing the instances in a map, and returning the same instance to each request for an instance with same parameters (multiton pattern)

Using lazy initialization to instantiate the object the first time it is requested (lazy initialization pattern)

Actionscript 3

The following is an example of a class with Lazy initialization implemented in Actionscript:

Basic Usage:

C

In C, lazy evaluation would normally be implemented inside a single function, or a single source file, using static variables.

In a function:

Using a single source file instead allows the state to be shared between multiple functions, while still hiding it from non-related functions.

fruit.h:

fruit.c:

main.c:

C#

In .NET 4.0 Microsoft has included a Lazy class that can be used to do lazy loading. Below is some dummy code that does lazy loading of Class Fruit

Here is a dummy example in C#.

The Fruit class itself doesn't do anything here, The class variable _typesDictionary is a Dictionary/Map used to store Fruit instances by typeName.

A fairly straightforward 'fill-in-the-blanks' example of a Lazy Initialization design pattern, except that this uses an enumeration for the type

C++

Here is an example in C++.

Java

Here is an example in Java.

Output

Number of instances made = 1 Banana Number of instances made = 2 Banana Apple Number of instances made = 2 Banana Apple

JavaScript

Here is an example in JavaScript.

Output

Number of instances made: 1 Apple Number of instances made: 2 Apple Banana Number of instances made: 2 Apple Banana

PHP

Here is an example of lazy initialization in PHP 5:

Python

Here is an example in Python.

Ruby

Here 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.

Smalltalk

Here is an example in Smalltalk, of a typical accessor method to return the value of a variable using lazy initialization.

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.

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

Scala

Scala has built-in support for lazy variable initiation.

Crystal

Output:

Number of instances made: 1 Banana Number of instances made: 2 Banana Apple Number of instances made: 2 Banana Apple

References

Lazy initialization Wikipedia

(Text) CC BY-SA

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