Hi Alex,
First, I have to apologize as the current status of Bio++ is a bit messy: we are in the process of releasing version 3.00, featuring a major change in the phylogenetic classes.
While the documentation between v2 and v3 is still a bit mixed up, I would suggest to start with v3, as the old v2 api will at some point not be maintained anymore.
The new website is here:
https://biopp.github.io/, and some documentation is available on the new wiki (see link on the webpage).
Now, there seems to be a confusion: the bpp files are not C++ files. These are options for the programs from the BppSuite software, in particular bppML. This software allows using some of the features of the libraries (notably model fitting), without the need of coding in C++. If you would like to see some code examples, you may have a look at the bpp-seq-example and bpp-phyl-example repositories on github.
Hope this helps a bit...
Julien.