Lazy initialization

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 instantiation 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 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.

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

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

Basic Usage:

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:

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#.

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