The template implementation is still new. The implementation in 2.4.1 represents a considerable improvement over that of previous releases, but it has a long way to go. This continues to improve from release to release.
g++ does not implement a separate pass to instantiate template functions
and classes at this point; for this reason, it will not work, for the most
part, to declare your template functions in one file and define them in
another. The compiler will need to see the entire definition of the
function, and will generate a static copy of the function in each file
in which it is used.
For 2.5.0, however, a new switch
-fexternal-templates was added; this makes it possible to have
only one globally visible copy of a given template expansion in your
executable. See the gcc manual for details.
Some features that the ANSI/ISO standardization committee has voted in
that don't appear in the ARM are supported, notably the mutable
keyword, in version 2.5.x.
As with any beta-test compiler, there are bugs. You can help improve the compiler by submitting detailed bug reports.
One of the weakest areas of g++ other than templates is the resolution of overloaded functions and operators in complex cases. The usual symptom is that in a case where the ARM says that it is ambiguous which function should be chosen, g++ chooses one (often the first one declared). This is usually not a problem when porting C++ code from other compilers to g++, but shows up as errors when code developed under g++ is ported to other compilers.
As of 2.5.0, the overloading code has been rewritten. For now, you must
specify the option -fansi-overloading to get the new code, since
there were some important users actually depending on g++'s incorrect
resolution of ambiguities. This switch should disappear in the future.
If a program that compiled under previous g++ versions now reports
that a use of an overloaded function is ambiguous, it is likely that the
old g++ was letting you write buggy code and the new one is detecting
the problem. If in doubt, consult the ARM.
[A full bug list would be very long indeed, so I won't put one here. I may add a list of frequently-reported bugs and "non-bugs" like the static class members issue mentioned above].