Looking for a basic PostGIS viewer to use as basis for own application

Although this isn't really the direct answer to your question, qgis acts as a relatively simple GIS viewer which can access PostGIS. In addition to the main application -- which is a relatively complex piece of software -- the core of the qgis library can be 'pulled out' and used from an application written in C/C++ or (I believe, with some work) Python.

Tim Sutton's post on the topic of building minimal qgis widgets is probably a good starting place for this. In order to build a simple widget for viewing a layer, the C code is only 63 lines, including headers + other boilerplate.

try SharpMap. Not written in C++ but in C#. Maybe it could be some use to you.

QGIS started out as a viewer for PostGIS, and is now a full blown open source GIS. From QGIS there is a very close integration with PostGIS enabling dynamic views, updateable views and a DB manager for working with spatial data building and running queries. Styles for your data can also be saved in PostGIS. Default styles can be set, so when you or another user opens a layer from scratch a default style is applied.

DB Manager:

SQL query builder: