QTextStream with stdout

[fyodor]

Hello. I am trying to do simple hello world console app with Qt. I use Qt 4.7.4 libraries and QtCreator 2.3.1 on Windows 7 x64 Home Premium SP1. When I build and run it nothing happens. Just a blank console appears. Here is code:

#include <QtCore/QCoreApplication>
#include <QString>
#include <QTextStream>
 
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QString str = "Hello World";
    QTextStream out(stdout, QIODevice::WriteOnly);
 
    out << str;
 
    return a.exec();
}

#include <QtCore/QCoreApplication>
#include <QString>
#include <QTextStream>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QString str = "Hello World";
    QTextStream out(stdout, QIODevice::WriteOnly);

    out << str;

    return a.exec();
}

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and .pro file:

#-------------------------------------------------
#
# Project created by QtCreator 2011-12-03T21:44:19
#
#-------------------------------------------------
 
QT       += core
 
QT       -= gui
 
TARGET = untitled
CONFIG   += console
CONFIG   -= app_bundle
 
TEMPLATE = app
 
 
SOURCES += main.cpp

#-------------------------------------------------
#
# Project created by QtCreator 2011-12-03T21:44:19
#
#-------------------------------------------------

QT       += core

QT       -= gui

TARGET = untitled
CONFIG   += console
CONFIG   -= app_bundle

TEMPLATE = app


SOURCES += main.cpp

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I tried on OSX Lion 10.7.2 but the result is same. So is it a bug or am I doing something wrong?

[Zlatomir]

That happens because you start an event-loop that just loops without doing nothing - i'm talking about the creation of that QCoreApplication and the call of exec().

For your "hello world" app you can skip the QCoreApplication and it's exec, so your main will look something like:

int main(int argc, char *argv[])
{
    QString str = "Hello World\n";
    QTextStream out(stdout, QIODevice::WriteOnly);
 
    out << str;
    //out.flush(); //a stream flush might be necessary to see the results immediately
    return 0;
}

int main(int argc, char *argv[])
{
    QString str = "Hello World\n";
    QTextStream out(stdout, QIODevice::WriteOnly);

    out << str;
    //out.flush(); //a stream flush might be necessary to see the results immediately
    return 0;
}

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[fyodor]

Thanks. It works. But Then why do we need QCoreApplication in console app if it can break something?

[Zlatomir]

You might need QCoreApplication if, for example, you code a "server"-side application that doesn't have a GUI but it waits for events (eg: some network requests from the "client"-side applications) - this is just the first example that crossed my mind, i'm sure there are other usages.