I'm a beginner in Qt and when I tried to use the connect method to bind the signal and slot, I referred to the examples I found the standard usage of. It is as:
connect(obj1, SIGNAL(signal(int)), obj2, SLOT(slot()))
However when I use as
connect(obj1, &Obj1::signal, obj2, &Obj2::slot)
It also worked well, thus a question occurs to me: What exactly did the macro SIGNAL and SLOT do? Did they just look for the signal in the class the object belongs to and return the address of it?
Then why do most programmers use these macros instead of using like &Obj1::signal as the latter appears to be simpler and don't need to edit the code when changing the parameter of the signals?
The use of the SIGNAL and SLOT macros used to be the only way to make connections, before Qt 5. The connection is made at runtime and require signal and slots to be marked in the header. For example:
Class MyClass : public QObject
{
Q_OBJECT
signals:
void Signal();
slots:
void ASlotFunction();
};
To avoid repetition, the way in which it works is described in the QT 4 documentation.
The signal and slot mechanism is part of the C++ extensions that are provided by Qt and make use of the Meta Object Compiler (moc).
This explains why signals and slots use the moc.
The second connect method is much improved as the functions specified can be checked at the time of compilation, not runtime. In addition, by using the address of a function, you can refer to any class function, not just those in the section marked slots:
The documentation was updated for Qt 5.
In addition, there's a good blog post about the Qt 4 connect workings here and Qt 5 here.
Addition to the first answer.
what exactly did the macro SIGNAL and SLOT do
Almost nothing. Look at the qobjectdefs.h:
# define SLOT(a) "1"#a
# define SIGNAL(a) "2"#a
It just adds 1 or 2. It means that next code is valid and works as expected:
QObject *obj = new QObject;
connect(obj,"2objectNameChanged(QString)",this,"1show()");//suppose this is a pointer to a QDialog subclass
obj->setObjectName("newNAme");
why do most programmers use these macros instead of using like
&Obj1::signal
- Because these macros work not only in Qt5.
- Because with these macros there is no complexity with overloaded
signals (it can make your code very dirty and it is really not a simple thing)
- Because with new syntax you sometimes need to use specific
disconnects
More details here.
To complete TheDarkKnight's answer, it is an excellent practice to refactor legacy code that is using the old Qt 4 SIGNAL and SLOT macros to Qt 5's new syntax using function address.
Suddenly, connection error will appear at compile time instead of at runtime! It's very easy to make a Qt 4 connection error as any spelling mistake will result in such an error. Plus, the name of the function must be the fully qualified name, i.e preceded with the full namespace if any.
Another benefit is the ability to use a lambda for the slot function, which can reduce need of a named function if the slot body is trivial.
