What's the difference between these params

Question

I'm new to C++, and I'm learning Qt.

Consider this line:

connect(ui->horizontalSlider, &QSlider::valueChanged,
        ui->progressBar, &QProgressBar::setValue);

What I don't understand is why you pass the address of a static (is it static?) method valueChanged ( &QSlider::valueChanged ) instead of the current object method address &ui->horizontalSlider->valueChanged . Although I can use this second option that works too.

Answer 1

You pass the address of the member function which should be called. The member function is not static though, that means it needs an object to work on.

class MyClass
{
    void aFunction();
}

here MyClass::aFunction is a member function.

What the compiler creates is similar to this

class MyClass
{
    static void aFunction(MyClass *this);
}

So whenever you call aFunction like my_instance.aFunction() the this pointer will be handed over implicitly, so the call basically becomes MyClass::aFunction(&my_instance) . As a result the address of aFunction is the same for every instance of MyClass . Yet to execute aFunction you need an instance of MyClass .

This is why in your case you have to provide connect with both the instance ui->horizontalSlider as well the function to be called on it &QSlider::valueChanged .

What I described is an oversimplification so take it with a grain of salt. Moreover when you have virtual functions things change.