BOOST threading: cout behavior

I'm new to Boost threading, and I am fixated on how output is executed from multiple threads. I have a simple boost :: thread counting from 9 to 1; the main thread waits and then prints "LiftOff .. !!"

#include <iostream>
#include <boost/thread.hpp>
using namespace std;

struct callable {
    void operator() ();
};

void callable::operator() () {
    int i = 10;
    while(--i > 0) {
        cout << "#" << i << ", ";
        boost::this_thread::yield();
    }
    cout.flush();
}

int main() {
    callable x;
    boost::thread myThread(x);

    myThread.join();

    cout << "LiftOff..!!" << endl;

    return 0;
}

The problem is that I have to use the explicit cout.flush () operator in my stream to output the output. If I do not use flush (), I get only "LiftOff !!". as a way out.

Can someone please advise why I need to use flush () explicitly?