c++ - 这个 C++ 反序列化习惯用法是什么？使用整数 ID 变量调用文件读取器函数作为 `reinterpret_cast<char *>(&id)?`

Question

我正在阅读内部的 SeqAn 库(它处理特定于生物学的文件格式和数据结构)，我遇到了我不太理解的 c++ 惯用语。

有一个唯一的 id 变量 record.rID，它是一个 __int32。指向它的指针被传递给另一个函数，该函数从文件中读取一堆数据并改变 id。

调用如下:

res = streamReadBlock(reinterpret_cast<char *>(&record.rID), stream, 4);

函数实现如下:

inline size_t
streamReadBlock(char * target, Stream<Bgzf> & stream, size_t maxLen)
{
    if (!(stream._openMode & OPEN_RDONLY))
        return 0;  // File not open for reading.

    // Memoize number of read bytes and pointer into the buffer target.
    size_t bytesRead = 0;
    char * destPtr = target;

    // Read at most maxLen characters, each loop iteration corresponds to reading the end of the first, the beginning of
    // the last or the whole "middle" buffers.  Of course, the first and only iteration can also only read parts of the
    // first buffer.
    while (bytesRead < maxLen)
    {
        // If there are no more bytes left in the current block then read and decompress the next block.
        int available = stream._blockLength - stream._blockOffset;
        if (available <= 0)
        {
            if (_bgzfReadBlock(stream) != 0)
                return -1;  // Could not read next block.
            available = stream._blockLength - stream._blockOffset;
            if (available <= 0)
                break;
        }

        // Copy out the number of bytes to be read or the number of available bytes in the next buffer, whichever number
        // is smaller.
        int copyLength = std::min(static_cast<int>(maxLen - bytesRead), available);
        char * buffer = &stream._uncompressedBlock[0];
        memcpy(destPtr, buffer + stream._blockOffset, copyLength);

        // Advance to next block.
        stream._blockOffset += copyLength;
        destPtr += copyLength;
        bytesRead += copyLength;
    }

    // If we read to the end of the block above then switch the block address to the next block and mark it as unread.
    if (stream._blockOffset == stream._blockLength)
    {
        stream._blockPosition = tell(stream._file);
        stream._blockOffset = 0;
        stream._blockLength = 0;
    }

    return bytesRead;
}

做一些跟踪，我可以看到 record.rID 在那里被分配，我猜 memcpy(destPtr, buffer + stream._blockOffset, copyLength); 发生在哪里，但我不太了解发生了什么以及如何分配有意义的记录 ID(但是我没有太多处理这种反序列化代码的经验)。

Answer 1

这是一种写入 int 的聪明方法。通过将 record.rID 的地址转换为指向 char 的指针，您可以使用 memcpy 直接向其写入字节。