[solved] QTimer in console application

[deleted28]

Hello,

can i use QTimer in a console application without QObject , Signal/ Slot overhead ?
I assume i need Signal Slot mechanism to get the timeout event directed to a function (Slot),
but I 'm not sure if there isn 't a simple solution.

thx wally

[Chris Kawa]

In short : No, you can't use a slot without using a slot :)

If you want to invoke some action after some time then you either need to sleep the thread (blocking) or wait for some time in a loop(non-blocking). Qt has both - thread support and an event loop. A connection from a QTimer is a form of using the second method so there's pretty little you can do in any other way, neither using standard facilities nor 3rd party libs.

Direct connections in a single threaded app are almost as cheap as plain function calls so the runtime overhead is next to none. What is your concern exactly?

[deleted28]

Thanks for reply.

bq. What is your concern exactly?

nothing special, just wnated a confirmation for my assumption above.
("i can not use QTimer efficiently in a single main.cpp without additional classfiles")
Actually i' m learning about QTimer and i think its a good class for playing around with QThread in next chapter. In the past i often met a much simplier solution as
my own approach after talking in forum. That's all

[Chris Kawa]

Oh, I thought you meant runtime efficiency not lines of code count.

bq. i can not use QTimer efficiently in a single main.cpp without additional classfiles

Sure you can. This app for example waits for 5s, does stuff, and then quits:
@
#include <QApplication>
#include <QTimer>

int main(int argc, char *argv[]) {
QApplication a(argc, argv);

QTimer t;
QObject::connect(&t, &QTimer::timeout, [&](){
    //do stuff
    a.exit();
});
t.start(5000);

return a.exec();

}
@

bq. Actually i’ m learning about QTimer and i think its a good class for playing around with QThread in next chapter.

QTimer is more of a single threaded construct. The timeout is handled(usually) in the thread the timer lives in. You can handle it in a worker thread of course but there's little point to it. If you want to wait for some time in a worker thread it's better to just sleep.

[Chris Kawa]

Actually starting with Qt 5.4 (yay, finally!) you can even shorten that to just:
@
#include <QApplication>
#include <QTimer>

int main(int argc, char *argv[]) {
QApplication a(argc, argv);

QTimer::singleShot(5000, [&](){
    //do stuff
    a.exit();
});

return a.exec();

}@

[hskoglund]

Even shorter: (couldn't resist nagging):
@
...
QTimer::singleShot(5000, [&]{
...
@

You don't need any parenthesis when lambda has no arguments, "learned that":http://qt-project.org/forums/viewreply/216748/ from JKSH the other day :-)

[deleted28]

Hello Chris,

that's what i was looking for, but i have still problems with it.

I'm on :
Qt Creator 3.3.0 (opensource)
Based on Qt 5.4.0 (GCC 4.6.1, 32 bit)

@//CK// #include <QApplication>
#include <QCoreApplication>
#include <QTimer>
#include <QDebug>

int main(int argc, char *argv[])
{

//CK// QApplication a(argc, argv);
QCoreApplication a(argc, argv);

QTimer::singleShot(5000, [&](){
    //do stuff

    qDebug() << "bingo";
    a.exit();
});

return a.exec&#40;&#41;;

}
@

erro message:
@.../main.cpp:25: error: no matching function for call to 'QTimer::singleShot(int, main(int, char**)::__lambda0)'
});
^@

[Chris Kawa]

@wally123: As I said that's a Qt 5.4 feature. Which Qt version are you using (not which version Qt Creator was build with) and which compiler? If you're not sure type QT_VERSION_STR in your code and press F2 to find out.

For gcc you need to add @CONFIG += c++11@ in your .pro file to have lambdas (but the error mentions lambda so you probably have that already).

@hskoglund: nice hint! on to the search/replace in my codebase... :)

[deleted28]

Chris,

bq. not which version Qt Creator was build with

sure , sorry
@
#define QT_VERSION_STR "5.4.0"@

Options/ Buile&Run: Kits, QT-versions reporting also 5.4.0
Yes gcc and adding to .pro file
@CONFIG += c++11@

made it work. Also thanks to hskoglund
Even it's a short code, it contains some interesting new stuff for me
to learn more about. especially this part:

@ &{ .... };@

thx wally

[deleted28]

The following is what i primarly intended to ask for :)

@#include <QCoreApplication>
#include <QTimer>
#include <QDebug>

void timer1_expired() {
qDebug() << "timer1 expired";
}

void timer2_expired() {
qDebug() << "timer2 expired";
}

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

QTimer::singleShot(4000, timer1_expired);
QTimer::singleShot(8000, timer2_expired);

qDebug() << "main";

return a.exec&#40;&#41;;

}@

[JKSH]

[quote author="hskoglund" date="1423561353"]You don't need any parenthesis when lambda has no arguments, "learned that":http://qt-project.org/forums/viewreply/216748/ from JKSH the other day :-)[/quote]Pay it forward! ;-)

[deleted28]

not sure it makes sense to add to an already solved topic (?)

in above lecture the main reason i did not solve the problem myself
by trial & error was Chris' hint to use
@CONFIG += c++11@

in .pro file.
It's mentioned here "docs":http://doc.qt.io/qt-5/qmake-variable-reference.html#config
but this is not really an explanation.
Where can i find an explanation what this instruction really effects
and in which cases to use ?

[JKSH]

[quote author="wally123" date="1423669903"]not sure it makes sense to add to an already solved topic (?)[/quote]That's perfectly ok if you are continuing to discuss the same topic.

[quote author="wally123" date="1423669903"]in above lecture the main reason i did not solve the problem myself
by trial & error was Chris' hint to use
@CONFIG += c++11@

in .pro file.
It's mentioned here "docs":http://doc.qt.io/qt-5/qmake-variable-reference.html#config
but this is not really an explanation.
Where can i find an explanation what this instruction really effects
and in which cases to use ?
[/quote]That instructions tells your compiler to enable "C++11":http://www.cprogramming.com/c++11/what-is-c++0x.html features.

The QTimer code that Chris posted uses a lambda expression, which was introduced in C++11; that's why you need that command.

[deleted28]

thx
needless to say which expression is now in center of interest :)
λ

[JKSH]

[quote author="wally123" date="1423671202"]thx
needless to say which expression is now in center of interest :)
λ [/quote]You're welcome! It's a very nice toy indeed. Once you learn how to use it, I'm sure you'll be using it lots. ;)

Lambda expressions make it much easier to write event-driven code in Qt. If your slot is only used in one place, you can define it as a lambda right in the "connect" statement. No need to create a separate function in your class anymore. See "Making Connections to Lambda Expressions":http://doc.qt.io/qt-5/signalsandslots-syntaxes.html#making-connections-to-lambda-expressions.

Happy coding!

[Chris Kawa]

The doc pretty much says everything there is to say about it but I'll try to expand.

As you may or may not know C++ goes through revisions and different compilers have varying support for the latest standard.
In case of GCC the default is c++03 and to enable the latest c++11 you need a compiler param -std=c++11. To make this portable across compilers qmake exposes this as the above CONFIG switch and does the right thing for the given compiler to enable c++11 support.

Lambdas (or captures or anonymous functions as they are also known) are new part of c++11 standard.
"Here":https://gcc.gnu.org/projects/cxx0x.html is a comprehensive table listing all the new features of c++11 and which gcc version supports what.

Oh, the next revision of c++ has been already voted in last year so expect something like CONFIG+=c++14 available pretty soon.

As for which c++11/14 features are supported/used by Qt you can travel the net, e.g. "here":http://woboq.com/blog/cpp11-in-qt5.html or "here":http://woboq.com/blog/cpp14-in-qt.html

[deleted28]

This forum is really incredible efficient for learning !

btw.: does anybody pay you for your time and sharing knowhow ?

[Chris Kawa]

Don't know about others but I pretty much operate on a "beerware":http://en.wikipedia.org/wiki/Beerware based rules.

[JKSH]

[quote author="wally123" date="1423672274"]This forum is really incredible efficient for learning ![/quote]It is. :) We hope that one day, when you're a Qt veteran, you'll come back and help newcomers too!

[quote author="wally123" date="1423672274"]btw.: does anybody pay you for your time and sharing knowhow ?[/quote]I suppose I get paid in pats-on-the-back, satisfaction, reputation, and knowledge + experience.

I often learn new things while doing research to answer a question. In turn, that helps me produce better code, which makes my customers happy, so they keep paying me.

BTW, it's not limited to this forum; the hacker community is full of volunteers, e.g. "StackOverflow":http://stackoverflow.com/

[deleted28]

:)