How Do I Connect Qt Signals and Slots?

In Qt, signals and slots are used for communication between objects. The signal/slot mechanism is a central feature of Qt and probably the part that differs most from other toolkits.

There are several ways to connect a signal to a slot:
QObject::connect() is the connection method generated by Qt’s meta-object system.
Functions can also be connected directly, with the static QObject::connect() function. The first parameter is the sender object, the second is the signal, and the third is the receiver object.

The fourth parameter is an optional context object which can be used to specify the receiving object’s thread affinity.
signals can also be connected to slots that are not member functions, by using lambdas or functors instead.

To connect a signal to a slot, we use the QObject::connect() method. This method has five parameters:
The first parameter is the sender object.
The second parameter is the SIGNAL macro of the sending object’s signal that we want to connect to.
The third parameter is the receiving object.

The fourth parameter is the SLOT macro of the receiving object’s slot that we want to connect to.
The fifth parameter is an optional connection type, which we will discuss later.
Now let’s see how this works with a simple example. We will create two QPushButton widgets, one with a label of “Click me” and one with a label of “Close”. When clicked, “Click me” button should emit a signal which will be connected to “Close” button’s slot so it closes the window:.

#include
#include

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

QPushButton button1(“Click me”); //Create first button
QPushButton button2(“Close”); //Create second button

//Connect button1’s clicked() signal to button2’s slot so it closes when clicked: (sender, SIGNAL(clicked()), receiver, SLOT(close())) (button1, SIGNAL(clicked()), &app, SLOT(quit())) (button1, SIGNAL(clicked()), button2 , SLOT(close())) (app = qApp)

QObject::connect(&button1, SIGNAL(clicked()), &app, SLOT(quit())); //sender ,SIGNAL(),receiver ,SLOT() ==>&app=qApp

QObject::connect(&button1, SIGNAL(clicked()), &button2 , SLOT(close()));//sender ,SIGNAL(),receiver ,SLOT();

button1.show(); //Show first button

return app.exec(); //Enter Qt event loop ==>int main(int argc=0 , char *argv[]=0) { . app.exec();} ==>(0)no argument ; (NULL)==’\0′ ;(&argv[0]=main()) ; (&main)=main address } }==>return app(*this , argc , argv).exec(); ==> while (! d->quitLockRefCount || d->aboutToQuitEmitted) { . } ==>return exec(); ==> while (! tryExit()) { processEvents(); } ==> bool tryExit () { bool doExit = d->canQuit; if (! doExit && ! qApp->topLevelWidgets().isEmpty()) { doExit = ! qApp->topLevelWidgets().first()-> close(); } return doExit; } ==> bool close () { bool f = true; if (parentWidget()) f = parentWidget()-> close(); else if (isWindow()) f = QWidget::close(); return f; } ==>(!f )==true ; return false } return true;} }==>(f )==false ; return false;} return true;} return d->close_helper(&result);} }==>(!result )=true ; return false }} return result;} }}==>(result)=false ; return false }}return exec_internal(flags);} int code = 0; if (! eventLoopRecursionGuard) code = qt_entry_interpreter(&argc , &argv); if ((code != 0 || ! d->canQuit) && ! aboutToQuitEmitted()) quitHandler(-255); quitDeferredFlag = false; cleanUpThreadData(); exit_loop_level–; if (exit_loop_level eventDispatcher) threadData->eventDispatcher->flushPostedEvents(); if (! eventLoopRecursionGuard++) processEventsUntilImplicitlyFlushed(); else –eventLoopRecursionGuard; exit_loop_level++; aboutToQuitEmittedValue++; exit_loop_level++; emit aboutToQuit(); aboutToQuitEmittedValue–; exit_loop_level–; d->canQuit = true; while (! tryExit()) {} quitDeferredFlag = false; cleanUpThreadData(); if (code != 0 && qAppName()) { fprintf(stderr ,”%s exited with code %d\n”, qPrintable(qAppName()) , code); } _exit((code > 255 ? 255 : code < 0 ? -code : code) & 255); }} _exit((255)); }} _exit((255));} _exit((255));} _exit((255));}.

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