Boost asio ConstBufferSequence - c++ 模板

Question

我希望获得有关 C++ 模板的一些指导。我一直在使用boost::asio 库通过 TCP 进行通信。到目前为止，我一直在使用 boost::asio 库中内置的存储容器。例如：

boost::array<char, 128> buf;
boost::system::error_code error;
size_t len = socket.read_some(boost::asio::buffer(buf), error);

要从套接字读取，我只需将boost::asio::buffer 类型的缓冲区包装在我的boost::array 对象周围。这工作正常，但现在我想在相反的方向工作。也就是说，我想写回套接字，从我已经拥有的一些自定义存储类中提取数据。 我的问题是，我如何理解 boost::asio::buffer 或更一般地由以下指定的参数类型所要求的模板类型要求：

template<typename ConstBufferSequence>
std::size_t send(   
   const ConstBufferSequence & buffers
);

API 列出了ConstBufferSequence 的要求，但我无法对此做出正面或反面。有人可以帮我理解吗？我要传递给“发送”函数的类型需要公开哪些方法？

Answer 1

boost::asio::buffer 返回实现ConstBufferSequence 和MutableBufferSequence 概念的对象；它不期望 你 来实现它们。您可以传递给buffer 的具体类型列在here。

Answer 2

虽然 boost::asio::buffer 工作正常，但如果您在内存中有一组连续的字节要用作缓冲区，如果您正在使用 scatter/gather I/O -- 或者特别是如果您是使用聚集写入（分散读取不太常见），您确实需要一个行为类似于 ConstBufferSequence 的对象。

[如果 Concepts 已将其纳入标准 [叹气]，您将需要一个实现 ConstBufferSequence 概念的对象]

下面是 QuickFAST (http:://www.quickfast.org) 中 Codecs::DataDestination 类的代码片段，它实现了 ConstBufferSequence：

class DataDestination
{
  /// @brief Support for asio gather writes: forward iterator through buffers
  ///
  /// The intent is for DataDestination to conform to the asio::ConstBufferSequence concept.
  /// This would allow it to be passed directly to the asio::write(v)
  /// Note the implication that if asynch writes are used the DataDestination must
  /// remain intact until the write completes.
  class const_iterator
  {
  public:
    /// @brief construct an iterator pointing into a DataDestination
    /// @param destination is the buffer-container
    /// @param position is the starting position for the iterator.
    const_iterator(const DataDestination & destination, size_t position)
      : destination_(destination)
      , position_(position)
    { 
    }
     /// @brief Point iterator to next buffer (preincrement)
    const_iterator & operator ++()
    {
      if(position_ < destination_.size())
      {
        ++position_;
      }
      return *this;
    }

    /// @brief Point iterator to next buffer (postincrement)
    const_iterator operator ++(int)
    {
      const_iterator result(*this);
      if(position_ < destination_.size())
      {
        ++position_;
      }
      return result;
    }

    /// @brief dereference the iterator to find the actual buffer
    boost::asio::const_buffer operator * () const
    {
      const WorkingBuffer & buffer(destination_[position_]);
      return boost::asio::const_buffer(buffer.begin(), buffer.size());
    }

    /// @brief dereference the iterator to find the actual buffer
    boost::asio::const_buffer operator -> () const
    {
      const WorkingBuffer & buffer(destination_[position_]);
        return boost::asio::const_buffer(buffer.begin(), buffer.size());
    }

    /// @brief compare iterators.
    /// @param rhs is the iterator to which this should be compared.
    bool operator == (const const_iterator & rhs) const
    {
      return position_ == rhs.position_;
    }

    /// @brief compare iterators.
    /// @param rhs is the iterator to which this should be compared.
    bool operator != (const const_iterator & rhs) const
    {
      return position_ != rhs.position_;
    }
  private:
    const_iterator & operator=(const_iterator &); // no autogenerated assignment

  private:
    const DataDestination & destination_;
    size_t position_;
  };

  /// @brief return iterator pointing to the first buffer.
  const_iterator begin()const
  {
    return const_iterator(*this, 0);
  }

  /// @brief return iterator pointing past the last buffer
  const_iterator end() const
  {
    return const_iterator(*this, used_);
  }
};