How to move std::ostringstream's underlying string object?

Question

#include <sstream>
#include <string>

using namespace std;

template<typename T>
string ToString(const T& obj)
{
    ostringstream oss;
    oss << obj;

    //
    // oss will never be used again, so I should
    // MOVE its underlying string.
    //
    // However, below will COPY, rather than MOVE, 
    // oss' underlying string object!
    //
    return oss.str();
}

How to move std::ostringstream's underlying string object?

Answer 1

The standard says that std::ostringstream::str() returns a copy.

One way to avoid this copy is to implement another std::streambuf derived-class that exposes the string buffer directly. Boost.IOStreams makes this pretty trivial:

#include <boost/iostreams/stream_buffer.hpp>
#include <iostream>
#include <string>

namespace io = boost::iostreams;

struct StringSink
{
    std::string string;

    using char_type = char;
    using category = io::sink_tag;

    std::streamsize write(char const* s, std::streamsize n) {
        string.append(s, n);
        return n;
    }
};

template<typename T>
std::string ToString(T const& obj) {
    io::stream_buffer<StringSink> buffer{{}};

    std::ostream stream(&buffer);
    stream << obj;
    stream.flush();

    return std::move(buffer->string); // <--- Access the string buffer directly here and move it.
}

int main() {
    std::cout << ToString(3.14) << '\n';
}