Stanfillirenfro. You need to look at how you are constructing your Essaie objects. Essaie is a QMainWindow but you are creating a new QMainWindow instance in the constructor. You then create a QPushButton that is parented to the Essaie object and don't use a layout.

As for the specific issue with the m_butonDisplay button, you do not show any code to do with this button or the construction or display of an associated window. It may be a simple problem with the connect() call (you should see a message in your debug window if this is the case) , or somewhere else, we cannot tell.

As an aside, I am a native English speaker with approximately 50 French words in my vocabulary but I don't have too much trouble with code in French unless extensive comments are required to understand what is going on.