libq++ and security help

Hi, I' d really appreciate some help with the following issue:

I'm using libq++ for a C++ frontend; the problem is I need to have
many users who don't have accounts on the server. I tried to find
information on connecting to the database specifying the username and
password, but it looks like the libq++ functions rely on the environment
variables.
I used mysql for a while and I had no problem doing this from
the C++ interface, and I know there has to be a way in postgresql, since
psql can do it. Could someone please point me into the right direction?
The pg_passwd man page seems to have something, but it only pertains to
tcl; I could find nothing in the libq++ examples , the user/programmer's
guides , or the mailing list archives. I looked in the header files also,
but maybe I missed something ... any ideas?

Thanks A LOT,

Monica