设置标准流使用的内部缓冲区 (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，方便数据的格式化.

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.

最初，我使用以下代码将流的内容复制到缓冲区中，但希望避免这种解决方案，因为它复制了太多数据.

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);
}

这是我发现 streambuf::pubsetbuf() 方法，简单地将上面的代码改写如下.

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;
}

不幸的是，这在 Visual Studio 2008 附带的 C++ 标准库实现下不起作用；buffer 保持不变.

Unfortunately, this does not work under the C++ standard library implementation that ships with Visual Studio 2008; buffer remains unchanged.

我查看了 pubsetbuf 的实现，结果发现它实际上什么都不做".

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);
}

这似乎是给定 C++ 标准库实现的限制.配置流以将其内容写入给定缓冲区的推荐方法是什么?

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?

推荐答案

在对这个问题进行更多研究并仔细检查我的代码后，我遇到了 一篇帖子 建议使用手动编码std::streambuf 类.这段代码背后的想法是创建一个 streambuf 来初始化它的内部以引用给定的缓冲区.代码如下.

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);
    }
};

现在，如果您查看我的原始代码，你会注意到我并不是真的需要一个 stringstream 开始.我真正需要的是一种使用 IOStream 库和std::ostream 是一种更好的寻址类型这个问题.顺便说一句，我怀疑这就是 array_sink 类型来自 Boost.IOStreams 已实施.

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.

这是使用我的 ostreambuf 类型的修改后的代码.

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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