The GNU C++ library sources have been specially formatted so that with the proper invocation of another tool (Doxygen), a set of HTML pages are generated from the sources files themselves. The resultant documentation is referred to as Source Level Documentation, and is useful for examining the signatures of public member functions for the library classes, finding out what is in a particular include file, looking at inheritance diagrams, etc.
The source-level documentation for the most recent releases can be viewed online:
"the latest collection" (For the main development tree; see the date on the first page.)
This generated HTML collection, as above, is also available for download in the libstdc++ snapshots directory at
<URL:ftp://gcc.gnu.org/pub/gcc/libstdc++/doxygen/>.
You will almost certainly need to use one of the
mirror sites to download
the tarball. After unpacking, simply load libstdc++-html-*/index.html
into a browser.
Documentation for older releases is available for download only, not online viewing.
In addition, an initial set of man pages are also available in the same place as the HTML collections. Start with C++Intro(3).