编写高效的C++程序方法之使用对象池

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        对象池技术可以避免在程序的生命期中创建和删除大量对象。如果知道程序需要同一类型的大量对象，而且对象的生命周期都很短，就可以为这些对象创建一个池（pool）进行缓存。只要代码中需要一个对象,就可以向对象池请求.用完此对象时，要把它放回池中。对象池只创建一次对象，因此它们的构造函数只调用一次，而不是每次使用时都调用。因此，当构造函数要完成一些设置动作，而且这些设置可以应用与该对象的多次使用时，对象池就很适用。另外在非构造函数方法调用中要在对象上设置特定于实例的参数时，也很适用采用对象池。

      一个对象池的实现

     这里提供了一个池类对象的模板的实现，你可以在自己的程序中使用这个池实现。这个池在构造时会分配一大块（chunk）指定类的对象（这里的“块”，可以理解为包括许多对象，即一堆对象），并通过 acquireObject( ) 方法交出对象。当客户用完这个对象时，会通过 releaseObject( ) 方法将其返回。如果调用了 acquireObject( ) ，但是没有空闲的对象，池会分配另外一块对象。

      对象池实现中最难的一方面是要记录哪些对象是空闲的，哪些对象正在使用。这个实现采用以下做法，即把空闲的对象保存在一个队列中。每次客户请求一个对象时，池就会把队列中第一个对象交个客户。这个池不会显式地跟踪正在使用的对象。它相信客户在用完对象后会正确的把对象交还到池中。另外，这个池在一个向量中记录所有已分配的对象。这个向量仅在撤销池时才会用到，以便是否所有对象的内存，从而避免内存泄露。

     以下是类的定义，要注意，这个模板是基于相应的类类型（要在池中构造何种类型的对象）参数化的。

#include <queue>
#include <vector>
#include <stdexcept>
#include <memory>

using std::queue;
using std::vector;

//
// template class ObjectPool
//
// Provides an object pool that can be used with any class that provides a
// default constructor.
//
// The object pool constructor creates a pool of objects, which it hands out
// to clients when requested via the acquireObject() method. When a client is
// finished with the object it calls releaseObject() to put the object back
// into the object pool.
//
// The constructor and destructor on each object in the pool will be called only
// once each for the lifetime of the program, not once per acquisition and release.
//
// The primary use of an object pool is to avoid creating and deleting objects
// repeatedly. The object pool is most suited to applications that use large 
// numbers of objects for short periods of time.
//
// For efficiency, the object pool doesn't perform sanity checks.
// Expects the user to release every acquired object exactly once.
// Expects the user to avoid using any objects that he or she has released.
//
// Expects the user not to delete the object pool until every object
// that was acquired has been released. Deleting the object pool invalidates
// any objects that the user had acquired, even if they had not yet been released.
//
template <typename T>
class ObjectPool
{
 public:
  //
  // Creates an object pool with chunkSize objects.
  // Whenever the object pool runs out of objects, chunkSize
  // more objects will be added to the pool. The pool only grows:
  // objects are never removed from the pool (freed), until
  // the pool is destroyed.
  //
  // Throws invalid_argument if chunkSize is <= 0.
  //
  ObjectPool(int chunkSize = kDefaultChunkSize)
    throw(std::invalid_argument, std::bad_alloc);

  //
  // Frees all the allocated objects. Invalidates any objects that have
  // been acquired for use.
  //
  ~ObjectPool();

  //
  // Reserve an object for use. The reference to the object is invalidated
  // if the object pool itself is freed.
  // 
  // Clients must not free the object!
  //
  T& acquireObject();

  //
  // Return the object to the pool. Clients must not use the object after
  // it has been returned to the pool.
  //
  void releaseObject(T& obj);

 protected:
  //
  // mFreeList stores the objects that are not currently in use
  // by clients.
  //
  queue<T*> mFreeList;
  //
  // mAllObjects stores pointers to all the objects, in use
  // or not. This vector is needed in order to ensure that all
  // objects are freed properly in the destructor.
  //
  vector<T*> mAllObjects;

  int mChunkSize;
  static const int kDefaultChunkSize = 10;

  //
  // Allocates mChunkSize new objects and adds them
  // to the mFreeList.
  //
  void allocateChunk();
  static void arrayDeleteObject(T* obj);

 private:
  // Prevent assignment and pass-by-value
  ObjectPool(const ObjectPool<T>& src);
  ObjectPool<T>& operator=(const ObjectPool<T>& rhs);
};

template<typename T>
const int ObjectPool<T>::kDefaultChunkSize;

template <typename T>
ObjectPool<T>::ObjectPool(int chunkSize) throw(std::invalid_argument,
    std::bad_alloc) : mChunkSize(chunkSize)
{
    if (mChunkSize <= 0) {
        throw std::invalid_argument("chunk size must be positive");
    }
    // Create mChunkSize objects to start
    allocateChunk();
}

//
// Allocates an array of mChunkSize objects because that's
// more efficient than allocating each of them individually.
// Stores a pointer to the first element of the array in the mAllObjects
// vector. Adds a pointer to each new object to the mFreeList.
//
template <typename T>
void ObjectPool<T>::allocateChunk()
{
    T* newObjects = new T[mChunkSize];
    mAllObjects.push_back(newObjects);
    for (int i = 0; i < mChunkSize; i++) {
        mFreeList.push(&newObjects[i]);
    }
}

//
// Freeing function for use in the for_each algorithm in the
// destructor.
//
template<typename T>
void ObjectPool<T>::arrayDeleteObject(T* obj)
{
    delete [] obj;
}

template <typename T>
ObjectPool<T>::~ObjectPool()
{
    // free each of the allocation chunks
    for_each(mAllObjects.begin(), mAllObjects.end(), arrayDeleteObject);
}

template <typename T>
T& ObjectPool<T>::acquireObject()
{
    if (mFreeList.empty()) {
        allocateChunk();
    }
    T* obj = mFreeList.front();
    mFreeList.pop();
    return (*obj);
}

template <typename T>
void ObjectPool<T>::releaseObject(T& obj)
{
    mFreeList.push(&obj);
}

       对于这个类定义有几点需要强调。首先，要注意，对象是按引用获取和释放的，而不是按指针，这样可以避免客户通过指针管理或释放对象。接下来，注意对象池的用户通过模板参数来指定可以创建哪一个类的对象（即类名），通过构造函数指定分配的“块大小”。这个“块大小”控制着一次可创建的对象数。以下是定义 kDefaultChunkSize 的代码：

template<typename T>
const int ObjectPool<T>::kDefaultChunkSize;

     根据类定义,默认值 10 对于大多数使用来说可能都太小了.如果程序一次需要成千上万的对象,就应该使用一个更大、更适合的值。

     构造函数验证 chunkSize 参数，并调用 allocateChunk( ) 辅助方法来得到起始的对象分配。

template <typename T>
ObjectPool<T>::ObjectPool(int chunkSize) throw(std::invalid_argument,
    std::bad_alloc) : mChunkSize(chunkSize)
{
    if (mChunkSize <= 0) {
        throw std::invalid_argument("chunk size must be positive");
    }
    // Create mChunkSize objects to start
    allocateChunk();
}

     allocateChunk( ) 方法在连续地存储空间中分配 mChunkSize 个元素。它会在一个 mAllObjects vector 中存储对象数组的指针，并把各个对象压至 mFreeLlist queue。

//
// Allocates an array of mChunkSize objects because that's
// more efficient than allocating each of them individually.
// Stores a pointer to the first element of the array in the mAllObjects
// vector. Adds a pointer to each new object to the mFreeList.
//
template <typename T>
void ObjectPool<T>::allocateChunk()
{
    T* newObjects = new T[mChunkSize];
    mAllObjects.push_back(newObjects);
    for (int i = 0; i < mChunkSize; i++) {
        mFreeList.push(&newObjects[i]);
    }
}

     析构函数只是释放 allocateChunk( ) 中分配的所有对象数组。不过，它使用了 for_each( ) STL算法来做到这一点，在此向 for_each( ) 传递一个arrayDelete( ) 静态方法的指针，这个方法会对各个对象数组具体完成删除调用。

//
// Freeing function for use in the for_each algorithm in the
// destructor.
//
template<typename T>
void ObjectPool<T>::arrayDeleteObject(T* obj)
{
    delete [] obj;
}

template <typename T>
ObjectPool<T>::~ObjectPool()
{
    // free each of the allocation chunks
    for_each(mAllObjects.begin(), mAllObjects.end(), arrayDeleteObject);
}

     acquireObject( ) 会返回空闲列表中的队头对象，如果没有空闲对象则首先调用 allocateChunk( ) 。

template <typename T>
T& ObjectPool<T>::acquireObject()
{
    if (mFreeList.empty()) {
        allocateChunk();
    }
    T* obj = mFreeList.front();
    mFreeList.pop();
    return (*obj);
}

     最后，releaseObject( ) 将对象返回到空闲列表的队尾。

template <typename T>
void ObjectPool<T>::releaseObject(T& obj)
{
    mFreeList.push(&obj);
}

     
使用对象池

     请考虑一个要从用户得到请求并处理这些请求的应用。这个应用很可能是图形前端和后端数据库之间的一个中间件。例如，这可能是一个航空预定系统或一个在线银行应用的一部分。你可能想把每个用户请求编码到一个对象中，这个类可能如下。

class UserRequest
{
public:
  UserRequest() {}
  ~UserRequest() {}

  // Methods to populate the request with specific information.
  // Methods to retrieve the request data.
  // (not shown)
         
protected:
  // data members (not shown)
};

     这里不用在程序的整个生命期中创建和删除大量请求,而是可以使用一个对象池。程序可能如下所示：

UserRequest& obtainUserRequest(ObjectPool<UserRequest>& pool)
{
  // Obtain a UserRequest object from the pool
  UserRequest& request = pool.acquireObject();

  // populate the request with user input
  // (not shown)

  return (request);
}

void processUserRequest(ObjectPool<UserRequest>& pool, UserRequest& req)
{
  // process the request
  // (not shown)

  // return the request to the pool
  pool.releaseObject(req);
}

int main(int argc, char** argv)
{
  ObjectPool<UserRequest> requestPool(1000);

  // Set up program
  // (not shown)

  while (true /* program is running */) {
    UserRequest& req = obtainUserRequest(requestPool);
    processUserRequest(requestPool, req);
  }

  return (0);
}

     另外，，，使用线程池也是一个提高C++程序效率的不错方式。线程池和对象池很相似，即不在程序的整个生命期中动态地创建和删除线程，而是创建一个线程池，按需使用池中的线程。如果程序要处理到来的网络请求,这种程序中常常会用到这种技术.web 服务器就可以维护一个线程池，以备查找页面，从而对到来的各个客户请求作出反应。