Question

To make a function of a class to be a slot, the class has to inherit from QObject. However, QObject takes up a quite large amount of memory. I am not sure how much it is and if the memory is for each class or each object. My code has many small data whose functions can be a slot sometime. I am wonder if there is a way to make a function of class to be a slot temporally when using it. After using it, the memory for the slot cost will be deleted. The following code illustrates the requirement.

class SmallData // size of 2 or 3 integers.
{
public:
    virtual void F(); // use it as a slot.
    virtual QMenu* createMenu(); // use this to create the context menu with
                                 // an action connected to F()
    ...
};

// use the small data
vector<SmallData> vec(1000000); // the vector is put at a tree view. When an
                                // item in the tree view is selected, a context
                                // menu pop up with an action to run F().
SmallData* data = treeView.selectedItem();    
connect(action, SIGNAL(triggered()), data, SLOT(F())); // How to make F() to be
                                                       // a slot just here.
                                                       // The action is from 
                                                       // data->createMenu().

Answer 1

If you can use Qt5, you can connect signals to plain functions and static methods (which essentially are funnily named plain functions):

connect(action, &QAction::triggered, 
        &SmallData::statF); 

Where action is a QAction instance, and SmallData::statF is a static method of SmallData.

Edit per Christian Rau’s comment, to call a particular instance, you can also connect to lambda:

connect(action, &QAction::triggered,
        [data]() { data->F(); });

Already with Qt4, you can use QSignalMapper to achieve much the same effect, with a few more objects. It allows you to add add a parameter (in this case, probably an integer index to your vec) to signal, based on which object emitted it. But in Qt4, receiver must still always be a QObject.