haiku-beta6/docs/user/support/BlockCache.dox

/*
 * Copyright 2007, Haiku, Inc. All Rights Reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *   Niels Sascha Reedijk <[email protected]>
 *
 * Proofreading:
 *   David Weizades <[email protected]>
 *   Thom Holwerda <[email protected]>
 * 
 * Corresponds to:
 *   /trunk/headers/os/support/BlockCache.h  rev 19972
 *   /trunk/src/kits/support/BlockCache.cpp  rev 4568
 */
 
 /*!
  \file BlockCache.h
  \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.
*/