How Can I Efficiently Write to a Pre-Allocated Buffer Using C Standard Streams?

Setting Internal Buffer for Standard Streams Using pubsetbuf

Problem:

In an attempt to write data to an existing buffer using a stringstream, a programmer initially used copy to transfer the stream contents to the buffer, intending to avoid excessive data copying. Subsequently, they tried streambuf::pubsetbuf() to achieve this, but found that it did not modify the buffer under the Visual Studio 2008 C standard library implementation.

Answer:

Upon further investigation, it became apparent that the pubsetbuf method in the given library implementation was essentially a no-op. To configure a stream to write to a specific buffer, utilizing a custom std::streambuf class is recommended:

template <typename char_type>
struct ostreambuf : public std::basic_streambuf<char_type, std::char_traits<char_type>> {
    ostreambuf(char_type* buffer, std::streamsize bufferLength) {
        setp(buffer, buffer + bufferLength);
    }
};

This custom stream buffer initializes its internals to reference the provided buffer. To use it:

ostreambuf<char> ostreamBuffer(buffer, size);
std::ostream messageStream(&ostreamBuffer);

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

This approach allows for direct writing to an external buffer using the IOStream library and std::ostream, circumventing the unnecessary copying involved in the initial copy method.

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