[solved] QTimer in console application
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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?
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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 -
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.
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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();}@
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Even shorter: (couldn't resist nagging):
@
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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 :-)
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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();}
@erro message:
@.../main.cpp:25: error: no matching function for call to 'QTimer::singleShot(int, main(int, char**)::__lambda0)'
});
^@ -
@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... :)
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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
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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();}@
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[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! ;-)
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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 ? -
[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.
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[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!
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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
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Don't know about others but I pretty much operate on a "beerware":http://en.wikipedia.org/wiki/Beerware based rules.
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[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/
