QIcon being reutilized during creation of QAction, is that possible?

leonardoMB

The following code is part of my constructor

iconTitle = QIcon(":/a/a.png");
actionTitle = QString(tr("a"));
_ActionOne = new QAction(iconTitle, actionTitle, this);
iconTitle = QIcon(":/a/b.png");
actionTitle = QString(tr("b"));
_ActionTwo = new QAction(iconTitle, actionTitle, this);

I would like to know Why the code above works. I mean, the iconTitle is passed as reference and we change it. So all Actions refer to the same memory, right?

JonB

@leonardoMB
Here's my 2 cents, based on intuition & superstition :)

When you pass the const QIcon & to QAction it increments a reference count, for the icon, not the iconTitle variable. Changing iconTille to (effectively "point to") a different QIcon won't have any effect on, say, your QAction call.

ChrisW67

You pass the QIcon by const reference and the QAction constructor makes a copy. That copy is not connected to the original after that. The same thing is happening with the title text.

A copy is required because the QAction object has to control/ensure the lifetime of the icon it may providing for display. For example, you can call the constructor like this:

_ActionOne = new QAction(iconTitle, QIcon(":/a/a.png"), this);

where the temporary QIcon has no existence beyond this statement, but the QAction must still function in a tool bar it is added to.

The Qt internals are designed to make such copies as efficient as they can be.

leonardoMB

@ChrisW67
But isn't it an assignment operator instead of a copy constructor call?

JonB

@leonardoMB
Here's my 2 cents, based on intuition & superstition :)

When you pass the const QIcon & to QAction it increments a reference count, for the icon, not the iconTitle variable. Changing iconTille to (effectively "point to") a different QIcon won't have any effect on, say, your QAction call.

leonardoMB

@JonB You are right, I understood it from the source code, right now and you reinforced it, thanks