haiku/docs/user/support/BlockCache.dox

/*
 * Copyright 2007 Haiku, Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk, niels.reedijk@gmail.com
 *
 * Proofreading:
 *		David Weizades, ddewbofh@hotmail.com
 *		Thom Holwerda, slakje@quicknet.nl
 *
 * Corresponds to:
 *		headers/os/support/BlockCache.h  rev 19972
 *		src/kits/support/BlockCache.cpp  rev 4568
 */


 /*!
	\file BlockCache.h
	\ingroup support
	\ingroup libbe
	\brief Implements a mechanism to store and retrieve memory blocks.
*/


/*!
	\var B_OBJECT_CACHE
	\brief Used in the constructor of BBlockCache. Determines that objects will
		be created using \c new[] and \c delete[].
*/


/*!
	\var B_MALLOC_CACHE
	\brief Used in the constructor of BBlockCache. Determines that objects will
		be created using \c malloc() and \c free().
*/


/*!
	\class BBlockCache
	\ingroup support
	\ingroup libbe
	\brief A class that creates and maintains a pool of memory blocks.

	In some performance critical code there might come a time where you require
	a lot of little blocks of memory that you want to access and dispose of
	continuously. Since allocating and freeing memory are 'expensive'
	operations, it is better to have a pool of memory blocks at your disposal.
	Luckily, the Haiku API provides a class that will act as the administrator
	of your memory pool, so you will not have to reinvent the wheel every time.

	The principle is easy. The constructor takes the number of blocks you
	want to create beforehand, the size of the blocks, and the method of
	allocation. This can either be #B_OBJECT_CACHE or #B_MALLOC_CACHE.
	The first one uses C++ operators \c new[] and \c delete[], while the second
	one uses \c malloc() and \c free(). Unless you have specific demands on
	performance or you want to take care of freeing the objects yourself, either
	way works fine.

	As soon as you have the memory pool, you can Get() blocks. If the
	pre-allocated memory blocks run out, BBlockCache will allocate new ones, so
	you will not have to worry about availability. As soon as you are done you
	can Save() the memory back into the pool. BBlockCache will make sure that no
	more blocks will be saved than the initial number you requested when you
	created the object, so be aware of that.

	As soon as you got a pointer from the Get() method, you own that block of
	memory; this means that you have the liberty to dispose of it yourself. It
	also means that when you delete your BBlockCache instance, any blocks of
	memory that are checked out will not be destroyed. In case you might want to
	delete your objects yourself, make sure you free the memory the right way.
	If you created the object as #B_OBJECT_CACHE, use \c delete[] to free your
	object. If you created the object as #B_MALLOC_CACHE, use \c free(). Please
	note that it defeats the purpose of this class if your are going to free all
	the objects yourself since it basically means that when the pool runs out,
	Get() will be allocating the objects by itself.

	\note BBlockCache is thread-safe.
*/


/*!
	\fn BBlockCache::BBlockCache(uint32 blockCount, size_t blockSize, uint32
		allocationType)
	\brief Allocate a new memory pool.

	\param blockCount The number of free memory blocks you want to allocate
		initially. This number is also used as the maximum number of free blocks
		that will be kept.
	\param blockSize The size of the blocks.
	\param allocationType Either #B_OBJECT_CACHE for using \c new[] and
		\c delete[] or #B_MALLOC_CACHE for \c malloc() and \c free().
*/


/*!
	\fn BBlockCache::~BBlockCache()
	\brief Destroy the empty blocks in the free list.

	Note that the blocks you checked out with Get() and not checked back in with
	Save() will not be freed, since ownership belongs to you. Make sure you
	clean up after yourself.
*/


/*!
	\fn void *BBlockCache::Get(size_t blockSize)
	\brief Get a block from the pool of free blocks.

	If the pool runs out of free blocks, a new one will be allocated. Please
	note that if the size given in the \c blockSize parameter is different from
	the size given in the constructor, a new block of memory will be created.
	Only sizes that match the blocks in the memory pool will come from the pool.

	\param blockSize The required size of the memory block.
	\return Returns a pointer to a memory block, or \c NULL if locking the
		object failed.
*/


/*!
	\fn void BBlockCache::Save(void *pointer, size_t blockSize)
	\brief Save a block of memory to the memory pool.

	The block of memory will only be added to the pool if the \c blockSize is
	equal to the size the object was created with and if the maximum number of
	free blocks in the list will not be exceeded. If not, the memory will be
	freed.

	Note that it is perfectly valid to pass objects other than those you got
	from Get(), but please note that the way it was created conforms to the way
	memory is allocated and freed in this pool. Therefore, only feed blocks that
	were created with \c new[] if the allocation type is #B_OBJECT_CACHE.
	Likewise, you should only use objects allocated with \c malloc() when the
	allocation type is #B_MALLOC_CACHE.
*/