How Can I Directly Write to a User-Supplied Buffer Using C Standard Streams?

Interfacing with a User-Supplied Buffer in Standard Streams Using pubsetbuf()

In this programming context, we aim to directly write data to an existing buffer using standard I/O streams. Let's explore how to accomplish this with the pubsetbuf() method.

Understanding pubsetbuf()

The pubsetbuf() method in streambuf, which is the base class for stream buffers in the C standard library, allows us to provide an externally allocated buffer to the stream. However, in the Microsoft Visual Studio 2008 implementation of the standard library, this method doesn't perform any buffer assignment.

An Alternative Approach: Customizing streambuf

To circumvent this issue, we can create a custom streambuf implementation that initializes its internal pointers to point to the given buffer. Here's a templated streambuf that accomplishes this:

template <typename char_type>
struct ostreambuf : public std::basic_streambuf<char_type, std::char_traits<char_type>>
{
    ostreambuf(char_type* buffer, std::streamsize bufferLength)
    {
        // Initialize internal buffer pointers to the provided buffer
        setp(buffer, buffer + bufferLength);
    }
};

Implementing Buffer Writing

Utilizing our custom streambuf, we can now modify our code to write directly to the external buffer:

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

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

This approach effectively writes data to the given buffer without intermediate copying, as desired.

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