Using std::cout in Qt Gui - c++

Question

I have an application that when run through terminal, the user has the option between command-line mode or GUI mode.
There doesn't seem to be any output to the console at all when using std::cout. std::cout statements don't work in the main event loop.
I have added CONFIG += console to my .pro file.
For now, I have been using QTextStream() which works fine:
QTextStream(cout) << "Hello World" << std::endl;
My question is:
Why can I not use std::cout? Does this have something to do with Qt affecting input and output streams? I couldn't find any documentation in Qt's docs on this.
int main(int argc, char *argv[])
{
std::cout << argv[1] << std::endl; //This is being outputted.
//if(argc == 2 && !strcmp(argv[1],"-win")){
if(true){ //Just for this example's sake
QApplication a(argc, argv);
std::cout << "Hello" << std::endl; //This is not being ouputted.
MainWindow w;
w.show();
return a.exec();
}
else
{
qDebug() << "Console Mode.\n";
std::cout << "Console Mode.\n";
//Do stuff
}
}

Answer 1

This is not a Qt issue, but how std::cout works. You seem to blow up your std::cout in here:
std::cout << argv[1] << std::endl;
Your issue can be reproduced even with a simple program like this:
main.pro
TEMPLATE = app
TARGET = main
CONFIG -= qt
SOURCES += main.cpp
main.cpp
#include <iostream>
int main(int /*argc*/, char **argv)
{
std::cout << argv[1] << std::endl;
std::cout << "Hello stdout!" << std::endl;
if (std::cout.bad())
std::cerr << "I/O error while reading\n";
return 0;
}
Build and Run
Success: qmake && make && ./main foo
Failure: qmake && make && ./main
In your case argv[1] is nil and so this makes std::cout not to print anything more. I would suggest to either pass an argument all the time and/or check against argc with some help usage print. The best would be to use the builtin command line parser in QtCore these days.
You could ask why? Because it is undefined behavior. You can read the details from the documentation:
basic_ostream& operator<<( std::basic_streambuf<CharT, Traits>* sb);
After constructing and checking the sentry object, checks if sb is a null pointer. If it is, executes setstate(badbit) and exits.
If you happen to have an issue with the IDE itself, for instance QtCreator, then follow these steps in case of QtCreator:
Projects -> Select a kit -> Run tab -> Run section -> Arguments

Answer 2

Works OK for me:
QT += core
QT -= gui
TARGET = untitled
CONFIG += console
CONFIG -= app_bundle
TEMPLATE = app
SOURCES += main.cpp
main.cpp:
#include <QCoreApplication>
#include <QTextStream>
#include <iostream>
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
std::cout << "Hello World" << std::endl;
return a.exec();
}
EDIT:
#include <QCoreApplication>
#include <QTextStream>
#include <QtWidgets/QWidget>
#include <QDebug>
#include <iostream>
int main(int argc, char *argv[]) {
std::cout << "test" << std::endl; // <--- THE PROBLEM IS HERE...IF YOU TRY A SIMPLE STRING IT WORKS FINE SO THE PROBLEM IS argv[1] IS AN EMPTY STRING
//if(argc == 2 && !strcmp(argv[1],"-win")){
if(true){
//Just for this example's sake
QCoreApplication a(argc, argv);
std::cout << "Hello" << std::endl; //This is not being ouputted.
return a.exec();
}
else
{
qDebug() << "Console Mode.\n";
std::cout << "Console Mode.\n";
//Do stuff
} }