c++ - 在 C++ 中，系统如何实现缓冲流？

Question

有人可以写一些示例代码来解释这个概念吗？我知道缓冲流的用途，但我也想知道如何实现它。

提前致谢！

Answer 1

您可以查看平台的实现、C++ 标准或Angelika Langer 和 Klaus Kreft 的“标准 C++ IOstreams and Locales”。

为相当长的学习曲线做好准备。流是古老而复杂的事情。（Francis Glassborow：“我毫不怀疑 I/O 库是任何语言中最困难的方面之一。”）

Answer 2

Veeeery 示意性地，对于“输入”流：

class BufferedInputStream
{
public:
  BufferedInputStream(SomeExternalDevice d)
  : m_Device(d),
    m_Current(m_Buffer),
    m_End(m_Buffer)
  {}

  char get(){
    if (!IsDataAvailableInBuffer()){
      ReadAChunkFromDiskAndPutItInBuffer();
    }
    return PopByteFromBuffer();
  }

private:

  bool IsDataAvailableInBuffer()const{
    return m_Current != m_End;
  }

  void ReadAChunkFromDiskAndPutItInBuffer(){
    // Buffer must be empty
    assert(!IsDataAvailableInBuffer());

    // Read bytes from the device
    bytes_read = Read(m_Device, m_Buffer, BufferSize);

    // Reposition the "begin" and "end" pointers
    m_Current = m_Buffer;
    m_End = m_Buffer + bytes_read;
  }

  char PopByteFromBuffer(){
    assert(IsDataAvailableInBuffer());
    return *m_Current++;
  }

  // For example, an OS file handle
  SomeExternalDevice m_Device;

  // The buffer itself
  char m_Buffer[BufferSize];

  // Begin of buffer contents
  char* m_Current;

  // End of buffer contents
  char* m_End;
};

这样，数据以缓冲区大小的块从磁盘读取，并且大多数对“get()”的调用不必最终调用操作系统，因为它们可以简单地从缓冲区返回一个字节。

Answer 3

查看 sstream 和 sstream.tcc的 STL 实现（链接到 SGI STL 实现）。

基stringstream类是basic_stringstream，它实现了basic_iostream接口。

  // [27.7.4] Template class basic_stringstream
  /**
   *  @brief  Controlling input and output for std::string.
   *
   *  This class supports reading from and writing to objects of type
   *  std::basic_string, using the inherited functions from
   *  std::basic_iostream.  To control the associated sequence, an instance
   *  of std::basic_stringbuf is used, which this page refers to as @c sb.
  */

有basic_stringbuf派生自的基类basic_streambuf。这保存了缓冲区。

  // [27.7.1] template class basic_stringbuf
  /**
   *  @brief  The actual work of input and output (for std::string).
   *
   *  This class associates either or both of its input and output sequences
   *  with a sequence of characters, which can be initialized from, or made
   *  available as, a @c std::basic_string.  (Paraphrased from [27.7.1]/1.)
   *
   *  For this class, open modes (of type @c ios_base::openmode) have
   *  @c in set if the input sequence can be read, and @c out set if the
   *  output sequence can be written.
  */