Unbuffered I/O is not working

Question

I have this program that is supposed to disable buffering for std::cout. I want to print out what I've written to the output device, but when I print str nothing comes out.

#include <iostream>
#include <sstream>
#include <string>

int main()
{
    std::cout.rdbuf()->pubsetbuf(0, 0);
    std::cout.unsetf(std::ios::unitbuf);

    std::cout << "Hello, World\n";

    std::stringstream ss;
    ss << std::cout.rdbuf();

    std::string str{ss.str()};

    std::cout << str; // nothing
    // str.size() == 0
}

Answer 1

std::cout.rdbuf()->pubsetbuf(0, 0);

This doesn't necessarily do anything because cout isn't specified to use a std::filebuf.

std::cout.unsetf(std::ios::unitbuf);

This clears the unitbuf bit so I/O is not unbuffered. Calling setf instead should request unbuffered I/O as desired.

ss << std::cout.rdbuf();

This attempts to read cout so it will extract nothing.

Just relying on setf( std::ios::unitbuf ) works as expected:

#include <iostream>
#include <unistd.h>

int main() {
    std::cout.setf( std::ios::unitbuf );

    std::cout << "Hel";
    write( 1, "lo, wo", 6 );
    std::cout << "rld!\n";
}