提高C++性能的编程技术读书笔记6.3

固定大小对象的内存池

1.未解决问题,删除内存池报错问题

#include <iostream>
template<typename T>
class MemoryPool
{
    public:
    MemoryPool (size_t size = EXPANSION_SIZE);
    ~MemoryPool ();

    inline void* alloc(size_t size);
    inline void free(void* someElement);

    private:
    MemoryPool<T>* next;

    enum { EXPANSION_SIZE = 32 };

    void expandTheFreeList(int howMany = EXPANSION_SIZE);
};

template <typename T>
MemoryPool<T>::MemoryPool(size_t size)
{
    expandTheFreeList(size);
}

template <typename T>
MemoryPool<T>::~MemoryPool()
{
    MemoryPool<T>* nextPtr = next;
    for (nextPtr = next ; nextPtr != NULL; nextPtr = next)
    {
        next = next->next;
        delete [] nextPtr;
    }
}

template <typename T>
inline
void* MemoryPool<T>::alloc(size_t size)
{
    if (next == NULL)
    {
        expandTheFreeList();
    }
    MemoryPool<T> *head = next;
    next = head->next;

    return head;
}

template<typename T>
inline
void MemoryPool<T>::free(void *doomed)
{
    MemoryPool<T>* head = static_cast<MemoryPool<T>*>(doomed);

    head->next = next;
    next = head;
}

template<typename T>
void MemoryPool<T>::expandTheFreeList(int howMany)
{
    size_t size = (sizeof(T) > sizeof(MemoryPool<T>*)) ? sizeof(T):sizeof(MemoryPool<T>*);
    MemoryPool<T>* runner = static_cast<MemoryPool<T>*>(static_cast<void *>(new char[size]));

    next = runner;
    for (int i = 0; i < howMany; i++)
    {
        runner->next = static_cast<MemoryPool<T>*> (static_cast<void*>(new char[size]));
        runner = runner->next;
    }
    runner->next = NULL;
}

class Rational
{
    public:
    Rational (int a = 0,int b = 1) : n(a),d(b) {}

    void *operator new(size_t size) { return memPool->alloc(size); }
    void operator delete(void* doomed, size_t size)
    {
        memPool->free(doomed);
    }
    static void newMemPool() { memPool = new MemoryPool<Rational>; }
    static void deleteMemPool() { delete memPool;}

    private:
    int n;
    int d;
    static MemoryPool<Rational>* memPool;
};
MemoryPool<Rational> *Rational::memPool = NULL;

int main()
{
    Rational *array[1000];

    Rational::newMemPool();

    for (int j = 0; j < 500; j++)
    {
        for (int i = 0; i < 1000; i++)
        {
            array[i] = new Rational(i);
        }
        for (int i = 0; i < 1000; i++)
        {
            delete array[i];
        }
    }
    Rational::deleteMemPool();
    return 0;
}