Question

Rather than sending an actual pointer to a value, the value is cast to a pointer. I found these examples in the GUI interface code of a GTK program.

g_signal_connect (pastebutton[pane],
                  "clicked",
                  G_CALLBACK(on_paste_button_pressed),
                  (void*)((long)pane<<4));

In the above example, I am referring to the last parameter of g_signal_connect. When on_paste_button_pressed is called by GTK2, on_paste_button_pressed casts the user_data void pointer back like so:

int pane = ((long)user_data) >> 4;

Actually, I added this particular example to the code, but I based it upon what was already there. I added the bit-shifting so as to avoid warnings about casting. The program itself has four panes containing quite a number of widgets, the copy and paste buttons allow you to copy all the values from one pane to another.

Is this way of casting a value to a pointer address often used, and are there reasons why this should not be used?

edit:

The cast from an integer to a void pointer can also be achieved like so:

void* void_ptr = some_int - NULL;

Answer 1

It is used. It is used quite commonly when it does what’s required.

One reason not to use it is that theoretically pointer size might be smaller than the source integer size.

Another reason not to use it is that it allows you to pass only one piece of integer data to the callback. If in the future you’ll need to add another piece of data (or switch to a non-integer), you’ll have to locate and rewrite every single place where the callback makes access to the passed data. So, if there’s a chance that you’d have to extend the data in the future, it is better to create a struct (even if it holds just a single int at this time) and pass a pointer to that struct.

But if you are sure that you’ll never have to pass anything other than that single integer and that your integer fits into a void *, then this technique is not in any way broken.

P.S. Pedantically speaking, neither C nor C++ appear to have the roundtrip guarantee for integer-to-void *-to-integer conversion, i.e. they don’t guarantee that it will work and restore the original integral value.