CODEWITHSUNDEEP
c++qtqt5
AlainD
AlainD

Reputation: 6597

Using QTextStream stops std::cout working

Using Qt (v5.15.0) in Visual Studio 2019 with the Qt VS Tools extension installed. Simple console application. Below is a simple demonstration of the problem:

#include <iostream>
#include <QDebug>

void main()
{
    std::cout << "cout: Line 1\n";
    qDebug() << "qDebug: Line 2";
    std::cout << "cout: Line 3\n";
    QTextStream qts{stdout};
    qts << "QTextStream: Line 4\n";
    qts << "QTextStream: Line 5\n";
    qts.flush();
    qDebug() << "qDebug: Line 6";
    std::cout << "cout: Line 7\n";       // Not printed!
    qDebug() << "qDebug: Line 8";
}

Everything is printed except Line 7. Comment out the four QTextStream lines, and Line 7 is printed again.

This seems to be related to the QTextStream object "capturing" the standard output. How do I get the object to release stdout so that std::cout starts working again?

Upvotes: 1

Views: 263

Answers (1)

user
user

Reputation: 944

On my computer (visual studio community edition 2019 and QT 5.9.9) this compiles and prints every line including line 7. If you want to make sure the buffer is flushed at line 7 you can use :

std::cout << "cout: Line 7\n" << std::flush;

but you'd have to use it every line you want to be certain to be flushed. Alternatively, if you want the buffer to be flushed every time you call std::cout you could use unitbuf :

std::cout << std::unitbuf

When the unitbuf flag is set, the associated buffer is flushed after each insertion operation. On your system these 4 QTextStream lines have the side effect of not having line 7 flushed by default. On my system these lines don't have this side effect. Because line 7 was never guaranteed to be flushed in the first place, this really isn't a problem.

Upvotes: 1

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