SIGNAL & SLOT macros in Qt : what do they do?

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.