How to open another window when the user enter correct user name and password in c++ gtkmm?

I am writing a program in which the user is supposed to enter the login details . If he enters the correct login details then the second window will open ( this is the actual app ) .

I have created login window . ( It has main .cpp )

I have also created my app . ( It also has main.cpp )

But I do not know how to link them ?