QSocketNotifier doesn't work with std::cin

//main.cpp
#include <QCoreApplication>
#include <QSocketNotifier>
#include <stdio.h>
#include "obj.h"
#include <iostream>
int main( int argc, char *argv[] )
{
    QCoreApplication a( argc, argv );
    QSocketNotifier notifier( STDIN_FILENO, QSocketNotifier::Read );
    Obj o;
    QObject::connect( &notifier, SIGNAL( activated(int) ), &o, SLOT( onReadyRead(int) ) );
    std::cout << ">>";
    return a.exec();
}
// obj.cpp
void Obj::onReadyRead(int)
{
    std::string command;
    std::cin >> command;
    std::cout << std::endl << "command is: " << command << std::endl << ">>";
}