Many GNU projects, including GCC, license their documentation under the GNU Free Documentation License. There is a long-standing dispute between the Free Software Foundation and the Debian project, which considers the GFDL to be non-free, mostly because it can forbid some sections of the documentation from being modified.
For this reason, the GCC documentation is not included in the Debian distribution itself. It is, however, distributed by the Debian project as part of the “non-free” repository. That the non-free repository is officially supported by Debian but not officially part of Debian is the result of much debate and compromise inside the Debian project.
You can search on the Debian website for a package containing the file
g++.1 (select “packages that contain files whose names contain the keyword”, because the man page is actually compressed so the file name is
g++.1.gz). You'll find it in the gcc-VERSION-doc package, which is a meta-package in the contrib repository that depends on the gcc-VERSION-doc package in the non-free repository.
On your machine, make sure that you have the contrib and non-free repositories enabled. Your
/etc/apt/sources.list should contain a line like
deb http://ftp.debian.org/debian squeeze main non-free contrib
or multiple lines like
deb http://ftp.debian.org/debian squeeze main
deb http://ftp.debian.org/debian squeeze contrib
deb http://ftp.debian.org/debian squeeze non-free
If you modify
apt-get update as root afterwards. Once you have these repositories in your sources, you can use apt-file to search for the package containing a file by name. You need to run
apt-file update as root after installing the program or modifying the sources list.
apt-file search g++.1
Once you find what package the file you want is in (as indicated above, you want the
g++-doc package), use
apt-get install g++-doc or whatever your favorite package manager is.