Class MemoryPoolAllocator

Synopsis

#include <include/rapidjson/allocators.h>

template <typename BaseAllocator = CrtAllocator>
class MemoryPoolAllocator

Description

Default memory allocator used by the parser and DOM.

This allocator allocate memory blocks from pre-allocated memory chunks.

It does not free memory blocks. And Realloc() only allocate new memory.

The memory chunks are allocated by BaseAllocator, which is CrtAllocator by default.

User may also supply a buffer as the first chunk.

If the user-buffer is full then additional chunks are allocated by BaseAllocator.

The user-buffer is not deallocated by this allocator.

Template Parameters

BaseAllocator - the allocator type for allocating memory chunks. Default is CrtAllocator.

Note
implements Allocator concept

Mentioned in

Methods

MemoryPoolAllocator overloadConstructor with chunkSize.
MemoryPoolAllocator overloadConstructor with user-supplied buffer.
~MemoryPoolAllocatorDestructor.
CapacityComputes the total capacity of allocated memory chunks.
ClearDeallocates all memory chunks, excluding the first/user one.
FreeFrees a memory block (concept Allocator)
MallocAllocates a memory block. (concept Allocator)
operator!=Compare (inequality) with another MemoryPoolAllocator.
operator=
operator==Compare (equality) with another MemoryPoolAllocator.
ReallocResizes a memory block (concept Allocator)
SharedWhether the allocator is shared.
SizeComputes the memory blocks allocated.

Source

Lines 128-420 in include/rapidjson/allocators.h.

template <typename BaseAllocator = CrtAllocator>
class MemoryPoolAllocator {
    //! Chunk header for perpending to each chunk.
    /*! Chunks are stored as a singly linked list.
    */
    struct ChunkHeader {
        size_t capacity;    //!< Capacity of the chunk in bytes (excluding the header itself).
        size_t size;        //!< Current size of allocated memory in bytes.
        ChunkHeader *next;  //!< Next chunk in the linked list.
    };

    struct SharedData {
        ChunkHeader *chunkHead;  //!< Head of the chunk linked-list. Only the head chunk serves allocation.
        BaseAllocator* ownBaseAllocator; //!< base allocator created by this object.
        size_t refcount;
        bool ownBuffer;
    };

    static const size_t SIZEOF_SHARED_DATA = RAPIDJSON_ALIGN(sizeof(SharedData));
    static const size_t SIZEOF_CHUNK_HEADER = RAPIDJSON_ALIGN(sizeof(ChunkHeader));

    static inline ChunkHeader *GetChunkHead(SharedData *shared)
    {
        return reinterpret_cast<ChunkHeader*>(reinterpret_cast<uint8_t*>(shared) + SIZEOF_SHARED_DATA);
    }
    static inline uint8_t *GetChunkBuffer(SharedData *shared)
    {
        return reinterpret_cast<uint8_t*>(shared->chunkHead) + SIZEOF_CHUNK_HEADER;
    }

    static const size_t kDefaultChunkCapacity = RAPIDJSON_ALLOCATOR_DEFAULT_CHUNK_CAPACITY; //!< Default chunk capacity.

public:
    static const bool kNeedFree = false;    //!< Tell users that no need to call Free() with this allocator. (concept Allocator)
    static const bool kRefCounted = true;   //!< Tell users that this allocator is reference counted on copy

    //! Constructor with chunkSize.
    /*! \param chunkSize The size of memory chunk. The default is kDefaultChunkSize.
        \param baseAllocator The allocator for allocating memory chunks.
    */
    explicit
    MemoryPoolAllocator(size_t chunkSize = kDefaultChunkCapacity, BaseAllocator* baseAllocator = 0) : 
        chunk_capacity_(chunkSize),
        baseAllocator_(baseAllocator ? baseAllocator : RAPIDJSON_NEW(BaseAllocator)()),
        shared_(static_cast<SharedData*>(baseAllocator_ ? baseAllocator_->Malloc(SIZEOF_SHARED_DATA + SIZEOF_CHUNK_HEADER) : 0))
    {
        RAPIDJSON_ASSERT(baseAllocator_ != 0);
        RAPIDJSON_ASSERT(shared_ != 0);
        if (baseAllocator) {
            shared_->ownBaseAllocator = 0;
        }
        else {
            shared_->ownBaseAllocator = baseAllocator_;
        }
        shared_->chunkHead = GetChunkHead(shared_);
        shared_->chunkHead->capacity = 0;
        shared_->chunkHead->size = 0;
        shared_->chunkHead->next = 0;
        shared_->ownBuffer = true;
        shared_->refcount = 1;
    }

    //! Constructor with user-supplied buffer.
    /*! The user buffer will be used firstly. When it is full, memory pool allocates new chunk with chunk size.

        The user buffer will not be deallocated when this allocator is destructed.

        \param buffer User supplied buffer.
        \param size Size of the buffer in bytes. It must at least larger than sizeof(ChunkHeader).
        \param chunkSize The size of memory chunk. The default is kDefaultChunkSize.
        \param baseAllocator The allocator for allocating memory chunks.
    */
    MemoryPoolAllocator(void *buffer, size_t size, size_t chunkSize = kDefaultChunkCapacity, BaseAllocator* baseAllocator = 0) :
        chunk_capacity_(chunkSize),
        baseAllocator_(baseAllocator),
        shared_(static_cast<SharedData*>(AlignBuffer(buffer, size)))
    {
        RAPIDJSON_ASSERT(size >= SIZEOF_SHARED_DATA + SIZEOF_CHUNK_HEADER);
        shared_->chunkHead = GetChunkHead(shared_);
        shared_->chunkHead->capacity = size - SIZEOF_SHARED_DATA - SIZEOF_CHUNK_HEADER;
        shared_->chunkHead->size = 0;
        shared_->chunkHead->next = 0;
        shared_->ownBaseAllocator = 0;
        shared_->ownBuffer = false;
        shared_->refcount = 1;
    }

    MemoryPoolAllocator(const MemoryPoolAllocator& rhs) RAPIDJSON_NOEXCEPT :
        chunk_capacity_(rhs.chunk_capacity_),
        baseAllocator_(rhs.baseAllocator_),
        shared_(rhs.shared_)
    {
        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        ++shared_->refcount;
    }
    MemoryPoolAllocator& operator=(const MemoryPoolAllocator& rhs) RAPIDJSON_NOEXCEPT
    {
        RAPIDJSON_NOEXCEPT_ASSERT(rhs.shared_->refcount > 0);
        ++rhs.shared_->refcount;
        this->~MemoryPoolAllocator();
        baseAllocator_ = rhs.baseAllocator_;
        chunk_capacity_ = rhs.chunk_capacity_;
        shared_ = rhs.shared_;
        return *this;
    }

#if RAPIDJSON_HAS_CXX11_RVALUE_REFS
    MemoryPoolAllocator(MemoryPoolAllocator&& rhs) RAPIDJSON_NOEXCEPT :
        chunk_capacity_(rhs.chunk_capacity_),
        baseAllocator_(rhs.baseAllocator_),
        shared_(rhs.shared_)
    {
        RAPIDJSON_NOEXCEPT_ASSERT(rhs.shared_->refcount > 0);
        rhs.shared_ = 0;
    }
    MemoryPoolAllocator& operator=(MemoryPoolAllocator&& rhs) RAPIDJSON_NOEXCEPT
    {
        RAPIDJSON_NOEXCEPT_ASSERT(rhs.shared_->refcount > 0);
        this->~MemoryPoolAllocator();
        baseAllocator_ = rhs.baseAllocator_;
        chunk_capacity_ = rhs.chunk_capacity_;
        shared_ = rhs.shared_;
        rhs.shared_ = 0;
        return *this;
    }
#endif

    //! Destructor.
    /*! This deallocates all memory chunks, excluding the user-supplied buffer.
    */
    ~MemoryPoolAllocator() RAPIDJSON_NOEXCEPT {
        if (!shared_) {
            // do nothing if moved
            return;
        }
        if (shared_->refcount > 1) {
            --shared_->refcount;
            return;
        }
        Clear();
        BaseAllocator *a = shared_->ownBaseAllocator;
        if (shared_->ownBuffer) {
            baseAllocator_->Free(shared_);
        }
        RAPIDJSON_DELETE(a);
    }

    //! Deallocates all memory chunks, excluding the first/user one.
    void Clear() RAPIDJSON_NOEXCEPT {
        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        for (;;) {
            ChunkHeader* c = shared_->chunkHead;
            if (!c->next) {
                break;
            }
            shared_->chunkHead = c->next;
            baseAllocator_->Free(c);
        }
        shared_->chunkHead->size = 0;
    }

    //! Computes the total capacity of allocated memory chunks.
    /*! \return total capacity in bytes.
    */
    size_t Capacity() const RAPIDJSON_NOEXCEPT {
        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        size_t capacity = 0;
        for (ChunkHeader* c = shared_->chunkHead; c != 0; c = c->next)
            capacity += c->capacity;
        return capacity;
    }

    //! Computes the memory blocks allocated.
    /*! \return total used bytes.
    */
    size_t Size() const RAPIDJSON_NOEXCEPT {
        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        size_t size = 0;
        for (ChunkHeader* c = shared_->chunkHead; c != 0; c = c->next)
            size += c->size;
        return size;
    }

    //! Whether the allocator is shared.
    /*! \return true or false.
    */
    bool Shared() const RAPIDJSON_NOEXCEPT {
        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        return shared_->refcount > 1;
    }

    //! Allocates a memory block. (concept Allocator)
    void* Malloc(size_t size) {
        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        if (!size)
            return NULL;

        size = RAPIDJSON_ALIGN(size);
        if (RAPIDJSON_UNLIKELY(shared_->chunkHead->size + size > shared_->chunkHead->capacity))
            if (!AddChunk(chunk_capacity_ > size ? chunk_capacity_ : size))
                return NULL;

        void *buffer = GetChunkBuffer(shared_) + shared_->chunkHead->size;
        shared_->chunkHead->size += size;
        return buffer;
    }

    //! Resizes a memory block (concept Allocator)
    void* Realloc(void* originalPtr, size_t originalSize, size_t newSize) {
        if (originalPtr == 0)
            return Malloc(newSize);

        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        if (newSize == 0)
            return NULL;

        originalSize = RAPIDJSON_ALIGN(originalSize);
        newSize = RAPIDJSON_ALIGN(newSize);

        // Do not shrink if new size is smaller than original
        if (originalSize >= newSize)
            return originalPtr;

        // Simply expand it if it is the last allocation and there is sufficient space
        if (originalPtr == GetChunkBuffer(shared_) + shared_->chunkHead->size - originalSize) {
            size_t increment = static_cast<size_t>(newSize - originalSize);
            if (shared_->chunkHead->size + increment <= shared_->chunkHead->capacity) {
                shared_->chunkHead->size += increment;
                return originalPtr;
            }
        }

        // Realloc process: allocate and copy memory, do not free original buffer.
        if (void* newBuffer = Malloc(newSize)) {
            if (originalSize)
                std::memcpy(newBuffer, originalPtr, originalSize);
            return newBuffer;
        }
        else
            return NULL;
    }

    //! Frees a memory block (concept Allocator)
    static void Free(void *ptr) RAPIDJSON_NOEXCEPT { (void)ptr; } // Do nothing

    //! Compare (equality) with another MemoryPoolAllocator
    bool operator==(const MemoryPoolAllocator& rhs) const RAPIDJSON_NOEXCEPT {
        RAPIDJSON_NOEXCEPT_ASSERT(shared_->refcount > 0);
        RAPIDJSON_NOEXCEPT_ASSERT(rhs.shared_->refcount > 0);
        return shared_ == rhs.shared_;
    }
    //! Compare (inequality) with another MemoryPoolAllocator
    bool operator!=(const MemoryPoolAllocator& rhs) const RAPIDJSON_NOEXCEPT {
        return !operator==(rhs);
    }

private:
    //! Creates a new chunk.
    /*! \param capacity Capacity of the chunk in bytes.
        \return true if success.
    */
    bool AddChunk(size_t capacity) {
        if (!baseAllocator_)
            shared_->ownBaseAllocator = baseAllocator_ = RAPIDJSON_NEW(BaseAllocator)();
        if (ChunkHeader* chunk = static_cast<ChunkHeader*>(baseAllocator_->Malloc(SIZEOF_CHUNK_HEADER + capacity))) {
            chunk->capacity = capacity;
            chunk->size = 0;
            chunk->next = shared_->chunkHead;
            shared_->chunkHead = chunk;
            return true;
        }
        else
            return false;
    }

    static inline void* AlignBuffer(void* buf, size_t &size)
    {
        RAPIDJSON_NOEXCEPT_ASSERT(buf != 0);
        const uintptr_t mask = sizeof(void*) - 1;
        const uintptr_t ubuf = reinterpret_cast<uintptr_t>(buf);
        if (RAPIDJSON_UNLIKELY(ubuf & mask)) {
            const uintptr_t abuf = (ubuf + mask) & ~mask;
            RAPIDJSON_ASSERT(size >= abuf - ubuf);
            buf = reinterpret_cast<void*>(abuf);
            size -= abuf - ubuf;
        }
        return buf;
    }

    size_t chunk_capacity_;     //!< The minimum capacity of chunk when they are allocated.
    BaseAllocator* baseAllocator_;  //!< base allocator for allocating memory chunks.
    SharedData *shared_;        //!< The shared data of the allocator
};





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