Okay, to summarize the tools I know:
1. The latest, free (as in beer) version of MPW, available from Macintosh Garden. Should run under Classic.
Has a C compiler, and a horribly outdated C++ compiler. No C++ standard library.
2. The compilers and libraries from 1., but run from the Mac OS X command line using the MPW emulator:
https://github.com/ksherlock/mpw
3. CodeWarrior Pro 4 (hard to find). The C++ compiler is surprisingly decent for its time, but still horribly outdated. A standard library exists, but I doubt it really conforms to the C++98 standard.
4. Various older compilers might run under Classic, too.
5. My own
https://github.com/autc04/Retro68 does run on G4s with Tiger. The only way to get *modern* C99 and C++11 for old Macs. Also, definitely a less polished product than the other options. It works, but you may run into bugs and unfinished features at any time.