模板类类型大小

Question

我已经为循环缓冲区编写了一个模板类：

template <class T> class CRingBuffer { /* ... */ };

该类执行的某些操作依赖于对T 大小的准确评估。当T 是BYTE（即sizeof(T) == 1，检查）时，这似乎可以正常工作。但是，当我尝试使用T 是DWORD 的同一个类时，由于某种原因，sizeof(T) 的计算结果为 16。我上次检查时，双字是 4 个字节，而不是 16。有谁知道为什么这正在发生吗？谢谢。

附加信息

由于其专有性质，我无法发布所有代码，但这里是有问题的类声明和函数定义：

template <class T> class CRingBuffer
{
#pragma pack( push , 1 )                // align on a 1-byte boundary

typedef struct BUFFER_FLAGS_tag
{
    T * pHead;                          // Points to next buffer location to write
    T * pTail;                          // Points to next buffer location to read
    BOOL blFull;                        // Indicates whether buffer is full.
    BOOL blEmpty;                       // Indicates whether buffer is empty.
    BOOL blOverrun;                     // Indicates buffer overrun.
    BOOL blUnderrun;                    // Indicates buffer underrun.
    DWORD dwItemCount;                  // Buffer item count.
} BUFFER_FLAGS, *LPBUFFER_FLAGS;

#pragma pack( pop )                     // end 1-byte boundary alignment

    // Private member variable declarations
private:
    T * m_pBuffer;                      // Buffer location in system memory
    T * m_pStart;                       // Buffer start location in system memory
    T * m_pEnd;                         // Buffer end location in system memory
    BUFFER_FLAGS m_tFlags;              // Buffer flags.
    DWORD m_dwCapacity;                 // The buffer capacity.

    // CRingBuffer
public:
    CRingBuffer( DWORD items = DEFAULT_BUF_SIZE );
    ~CRingBuffer();

    // Public member function declarations
public:
    DWORD Add( T * pItems, DWORD num = 1, LPDWORD pAdded = NULL );
    DWORD Peek( T * pBuf, DWORD num = -1, DWORD offset = 0, LPDWORD pWritten = NULL );
    DWORD Delete( DWORD num, LPDWORD pDeleted = NULL );
    DWORD Remove( T * pBuf, DWORD num = 1, LPDWORD pRemoved = NULL );
    void Flush( void );
    DWORD GetItemCount( void );
    BYTE GetErrorStatus( void );

    // Private member function declarations
private:
    void IncrementHead( LPBUFFER_FLAGS pFlags = NULL );
    void IncrementTail( LPBUFFER_FLAGS pFlags = NULL );
};

template <class T> void CRingBuffer<T>::IncrementHead( LPBUFFER_FLAGS pFlags )
{
    ASSERT(this->m_pBuffer != NULL);
    ASSERT(this->m_pStart != NULL);
    ASSERT(this->m_pEnd != NULL);
    ASSERT(this->m_tFlags.pHead != NULL);
    ASSERT(this->m_tFlags.pTail != NULL);

    pFlags = ( pFlags == NULL ) ? &(this->m_tFlags) : pFlags;

    // Verify overrun condition is not set.
    if ( pFlags->blOverrun == FALSE )
    {
        pFlags->pHead += sizeof(T); // increament buffer head pointer
        pFlags->blUnderrun = FALSE; // clear underrun condition

        // Correct for wrap condition.
        if ( pFlags->pHead == this->m_pEnd )
        {
            pFlags->pHead = this->m_pStart;
        }

        // Check for overrun.
        if ( pFlags->pHead == pFlags->pTail )
        {
            pFlags->blOverrun = TRUE;
        }
    }
}

在执行IncrementHead的pFlags->pHead += sizeof(T);时会出现上述问题。

Answer 1

哦，毕竟这真的很简单:)

不知不觉中，pFlags->pHead += sizeof(T); 使用了指针算法。 pHead 是一个指向T 的指针，当你将它增加sizeof(T) 时，这意味着你将它向前移动了那么多T 类型的元素，而不是你想象的那么多字节。所以T 的大小是平方的。如果您的目标是将指针移动到缓冲区的下一个元素，则只需将其增加 1：pFlags->pHead += 1;