g++ is the
Free Software Foundation's
C++ compiler. g++ is a
frontend of the
gcc system---as such, it uses the same
backend, including optimisation, as does the
C compiler. Of course, some
C++ features, such as
templates,
exceptions, and
virtual member functions, will result in executable
bloat or
slowdown. But that's the price one pays for better
abstraction.
g++ was, for a long time, significantly behind standard C++. However, since the development (and re-merge) of egcs, g++ has gotten much better in, for example, its support of templates,
the STL, namespaces, and exceptions. GNU libstdc++-v3, still in development, promises to provide even better STL support.
g++ is by no means considered the best C++ compiler out there. However, it is one of the best free ones, and certainly one of the most popular in the Unix world.