设置标准流使用的内部缓冲区（pubsetbuf） [英] Setting the internal buffer used by a standard stream (pubsetbuf)

问题描述

我正在编写一个需要将数据写入现有缓冲区的子例程，我想使用 stringstream 类，以方便数据的格式化。

使用以下代码将流的内容复制到缓冲区，但是希望避免此解决方案，因为它复制了太多的数据。

  #include< sstream> 
 #include< algorithm> 
 
 void FillBuffer（char * buffer，unsigned int size）
 {
 std :: stringstream message; 
 message<< Hello<< std :: endl; 
 message<< 世界！ << std :: endl; 
 
 std :: string messageText（message.str（））; 
 std :: copy（messageText.begin（），messageText.end（），buffer）; 
} 
  

这是当我发现 streambuf :: pubsetbuf（） 方法，代码如下。

  #include< sstream> 
 
 void FillBuffer（char * buffer，unsigned int size）
 {
 std :: stringstream message; 
 message.rdbuf（） - > pubsetbuf（buffer，size）; 
 
 message<< Hello<< std :: endl; 
 message<< 世界！ << std :: endl;不幸的是，这并不适用于Visual C ++标准库下的C ++标准库实现，因为它不支持Visual C＃ Studio 2008;  buffer 保持不变。
 
 
 
我看过 pubsetbuf 
  virtual _Myt * __ CLR_OR_THIS_CALL setbuf（_Elem *，streamsize）
 {//向外部代理提供缓冲区（do nothing）
 return（this）; 
} 
  
这似乎是给定C ++标准库实现的限制。 
解决方案
在对这个问题进行一些更多的研究后， ，并仔细检查了我的代码，我遇到了帖子，建议使用手动编码的  std :: streambuf  类。这个代码的背后的想法是创建一个 streambuf ，它初始化它的内部来引用给定的缓冲区。代码如下。
  #include< streambuf> 
 
 template< typename char_type> 
 struct ostreambuf：public std :: basic_streambuf< char_type，std :: char_traits< char_type> > 
 {
 ostreambuf（char_type * buffer，std :: streamsize bufferLength）
 {
 //设置put指针是缓冲区的开始并记录它的长度。 
 setp（buffer，buffer + bufferLength）; 
} 
}; 
  
现在，如果您查看我原来的代码，你会注意到我真的不需要一个 stringstream 开始。我真正需要的是一种方法，使用 IOStream 库和  std :: ostream  是一个更好的类型来解决这个问题。顺便说一下，我怀疑这是 array_sink 类型来自 Boost.IOStreams 。
 
 
 
这是使用我的 ostreambuf 类型的修改后的代码。
  #include< ostream> 
 #includeostreambuf.h//从上面包含ostreambuf结构体的文件。 
 
 void FillBuffer（char * buffer，unsigned int size）
 {
 ostreambuf< char> ostreamBuffer（buffer，size）; 
 std :: ostream messageStream（& ostreamBuffer）; 
 
 messageStream<< Hello<< std :: endl; 
 messageStream<< 世界！ << std :: endl; 
} 
  
 
I'm writing a subroutine that needs to write data to an existing buffer, and I would like to use the stringstream class to facilitate the formatting of the data.

Initially, I used the following code to copy the contents of the stream into the buffer, but would like to avoid this solution as it copies too much data.
#include <sstream>
#include <algorithm>

void FillBuffer(char* buffer, unsigned int size)
{
    std::stringstream message;
    message << "Hello" << std::endl;
    message << "World!" << std::endl;

    std::string messageText(message.str());
    std::copy(messageText.begin(), messageText.end(), buffer);
}
This is when I discovered the streambuf::pubsetbuf() method and simply rewrote the above code as follows.
#include <sstream>

void FillBuffer(char* buffer, unsigned int size)
{
    std::stringstream message;
    message.rdbuf()->pubsetbuf(buffer, size);

    message << "Hello" << std::endl;
    message << "World!" << std::endl;
}
Unfortunately, this does not work under the C++ standard library implementation that ships with Visual Studio 2008; buffer remains unchanged.

I looked at the implementation of pubsetbuf and it turns out that it literally "does nothing".
virtual _Myt *__CLR_OR_THIS_CALL setbuf(_Elem *, streamsize)
{   // offer buffer to external agent (do nothing)
    return (this);
}
This appears to be a limitation of the given C++ standard library implementation.  What is the recommended way to configure a stream to write its contents to a given buffer?
解决方案
After some more research on this problem, and scrutiny of my code, I came across a post suggesting the use of a hand-coded std::streambuf class.  The idea behind this code is to create a streambuf that initializes its internals to refer to the given buffer.  The code is as follows.
#include <streambuf>

template <typename char_type>
struct ostreambuf : public std::basic_streambuf<char_type, std::char_traits<char_type> >
{
    ostreambuf(char_type* buffer, std::streamsize bufferLength)
    {
        // set the "put" pointer the start of the buffer and record it's length.
        setp(buffer, buffer + bufferLength);
    }
};
Now if you look at my original code, you will notice that I didn't really need a stringstream to begin with.  All I really needed was a way to write to an external buffer using the IOStream library and std::ostream is a much better type to address this problem.  Incidentally, I suspect this is how the array_sink type from Boost.IOStreams is implemented.

Here is the modified code that uses my ostreambuf type.
#include <ostream>
#include "ostreambuf.h"  // file including ostreambuf struct from above.

void FillBuffer(char* buffer, unsigned int size)
{
    ostreambuf<char> ostreamBuffer(buffer, size);
    std::ostream messageStream(&ostreamBuffer);

    messageStream << "Hello" << std::endl;
    messageStream << "World!" << std::endl;
}


                        
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