If you decide to proceed on this project there's no need to reinvent the wheel.
I use Netbeans for my development. it has quite a good facility for working with databases and I use it regularly with Postgres. Since Netbeans is now licensed under Apache 2 you might find useful code there. Be assured it uses JDBC for access but JDBC is universally available and the folks at Postgresql have done quite a nice job with JDBC drivers. Of course, this already works on all platforms. The implementation is basic but very useful: i.e. a good starting point.
Thank you for the point. I'm the C++ programmer and I'm author of the
C++ client library for PostgreSQL - Pgfe and I'm going to use it in this
project. But I'm not sure about the cross-platform GUI toolkit.