Provides ways to decrease object count thus improving application required objects structure. Flyweight pattern is used when we need to create a large number of similar objects.
Pattern type - Structural
- This pattern is useful when a very large number of instances of a class must be created.
- It allows to create huge number of virtual objects without need to explicitly instantiate every single one.
- Reduces memory footprint and speedups code execution
- simplifies code
- Useful when there are high costs of object storing
- ... and objects does not have to be unique
- works nicely with factory, visitor and null object pattern
Important note is that flyweight is dedicated to object which are immutable.
Each "flyweight" object is divided into two pieces: the state-dependent (extrinsic) part, and the state-independent (intrinsic) part.
Intrinsic state is stored (shared) in the Flyweight object.
Extrinsic state is stored or computed by client objects, and passed to the Flyweight when its operations are invoked.
Free list is a mechanism used in garbage collecting in some programming languages (Ruby).
Idea is that interpreter (process) pre-allocates some memory for objects to have ability to
instantiate/create new ones quickly.
In a given example new objects of IListObject type are created within FreeList.
During initialization given amount of not used IListObject are created and cached.
By calling method getNew() of FreeList object, reference count of IListObject is increased
and point to created object is returned.
If object is removed (by calling deleteObject() method) it's reference count is set to 0.
In fact object is not destroyed at all, but can be reused by FreeList if, however,
there are no free (reusable) objects in free list, allocation exception is raised.
Other example might be how number from range -5...256 are "pre cached" in Python's interpreter.