AllocationPool.h

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#pragma once





template <class T>
class cAllocationPool
{
public:
    class cStarvationCallbacks
    {
    public:
        virtual ~cStarvationCallbacks() {}

        virtual void OnStartUsingReserve() = 0;

        virtual void OnEndUsingReserve() = 0;

        virtual void OnOutOfReserve() = 0;
    };

    virtual ~cAllocationPool() {}

    virtual T * Allocate() = 0;

    virtual void Free(T * a_ptr) = 0;

    bool IsEqual(const cAllocationPool & a_Other) const NOEXCEPT
    {
        return ((this == &a_Other) || DoIsEqual(a_Other) || a_Other.DoIsEqual(*this));
    }

    friend bool operator == (const cAllocationPool & a_Lhs, const cAllocationPool & a_Rhs)
    {
        return a_Lhs.IsEqual(a_Rhs);
    }

    friend bool operator != (const cAllocationPool & a_Lhs, const cAllocationPool & a_Rhs)
    {
        return !a_Lhs.IsEqual(a_Rhs);
    }

private:
    virtual bool DoIsEqual(const cAllocationPool & a_Other) const NOEXCEPT = 0;
};





template <class T>
class cListAllocationPool:
    public cAllocationPool<T>
{
public:

    cListAllocationPool(std::unique_ptr<typename cAllocationPool<T>::cStarvationCallbacks> a_Callbacks, size_t a_MinElementsInReserve, size_t a_MaxElementsInReserve) :
        m_MinElementsInReserve(a_MinElementsInReserve),
        m_MaxElementsInReserve(a_MaxElementsInReserve),
        m_Callbacks(std::move(a_Callbacks))
    {
        for (size_t i = 0; i < m_MinElementsInReserve; i++)
        {
            void * space = malloc(sizeof(T));
            if (space == nullptr)
            {
                m_Callbacks->OnStartUsingReserve();
                break;
            }
            m_FreeList.push_front(space);
        }
    }


    virtual ~cListAllocationPool() override
    {
        while (!m_FreeList.empty())
        {
            free(m_FreeList.front());
            m_FreeList.pop_front();
        }
    }


    virtual T * Allocate() override
    {
        if (m_FreeList.size() <= m_MinElementsInReserve)
        {
            void * space = malloc(sizeof(T));
            if (space != nullptr)
            {
                #if defined(_MSC_VER) && defined(_DEBUG)
                    // The debugging-new that is set up using macros in Globals.h doesn't support the placement-new syntax used here.
                    // Temporarily disable the macro
                    #pragma push_macro("new")
                    #undef new
                #endif

                return new(space) T;

                #if defined(_MSC_VER) && defined(_DEBUG)
                    // Re-enable the debugging-new macro
                    #pragma pop_macro("new")
                #endif
            }
            else if (m_FreeList.size() == m_MinElementsInReserve)
            {
                m_Callbacks->OnStartUsingReserve();
            }
            else if (m_FreeList.empty())
            {
                m_Callbacks->OnOutOfReserve();
                // Try again until the memory is avalable
                return Allocate();
            }
        }
        // placement new, used to initalize the object

        #if defined(_MSC_VER) && defined(_DEBUG)
            // The debugging-new that is set up using macros in Globals.h doesn't support the placement-new syntax used here.
            // Temporarily disable the macro
            #pragma push_macro("new")
            #undef new
        #endif

        T * ret = new (m_FreeList.front()) T;

        #if defined(_MSC_VER) && defined(_DEBUG)
            // Re-enable the debugging-new macro
            #pragma pop_macro("new")
        #endif

        m_FreeList.pop_front();
        return ret;
    }


    virtual void Free(T * a_ptr) override
    {
        if (a_ptr == nullptr)
        {
            return;
        }

        a_ptr->~T();  // placement destruct.

        if (m_FreeList.size() >= m_MaxElementsInReserve)
        {
            free(a_ptr);
            return;
        }

        m_FreeList.push_front(a_ptr);
        if (m_FreeList.size() == m_MinElementsInReserve)
        {
            m_Callbacks->OnEndUsingReserve();
        }
    }

private:
    size_t m_MinElementsInReserve;
    size_t m_MaxElementsInReserve;
    std::list<void *> m_FreeList;
    std::unique_ptr<typename cAllocationPool<T>::cStarvationCallbacks> m_Callbacks;

    virtual bool DoIsEqual(const cAllocationPool<T> & a_Other) const NOEXCEPT override
    {
        return (dynamic_cast<const cListAllocationPool<T>*>(&a_Other) != nullptr);
    }
};