I am trying to connect QPushButton to lambda expression:
QPushButton* loadTextFileButton = new QPushButton("load");
connect(loadTextFileButton, &QPushButton::clicked, [](){
qDebug()<<"clicked";
});
Compiler gives me an errors like: No matching function to call "MyClass::connect(..."
What I am doing wrong?
The connect function, which is part of Qt's signal and slots mechanism is Qt's extension to C++.
The connect function is actually a static function of QObject, so you can use it from anywhere, by simply including QObject: -
#include <QObject>
...
QObject::connect(itemPtr1, SIGNAL(someFunction()), itemPtr2, SLOT(someOtherFunction());
The objects itemPtr1 and itemPtr2 are pointers to instances of classes that are derived from QObject and support signals and slots.
In order for a class to use the signal and slot mechanism, it must inherit from QObject and declare the Q_OBJECT macro:-
class MyClass : public QObject
{
Q_OBJECT // the Q_OBJECT macro, which must be present for signals and slots
public:
MyClass(QObject* parent);
signals:
public slots:
private:
void StandardFunction();
};
As this class inherits QObject, it now has direct access to the connect function, allowing calling connect directly:-
QPushButton* loadTextFileButton = new QPushButton("load");
connect(loadTextFileButton, &QPushButton::clicked, []()
{
qDebug()<<"clicked";
});
Finally, Qt 5 introduced a new syntax for signals and slots: -
connect(loadTextFileButton, &QPushButton::clicked, this, &MyClass::StandardFunction);
You may have noticed that the advantage here is that the signal can be connected to a function that is not declared as a slot. In addition, using this syntax provides compile time error checking.
connectis a static method of QObject. You can writeQObject::connect(...