[gtkmm] how to trigger an action when my mouse leaves a selected widget?

["Andreas B. Thun" <abt gmx de>]

Dear members,

maybe you can give me some hints for getting
me on the right track:

I want to trigger an action when my mouse leaves
a selected widget, an entry for example.

Scenario is: I enter text into the entry box
and I wanna check, if the entered value is
correct when the widget loses focus.


TIA,
Andi