If I get some C++ code built by, lets say, GCC 4.8 on Ubuntu, the code has no GUI/interface, only call standard Linux libraries, then can the binary run on RHEL 5/6, with mu
The issue is probably more Glibc than libstdc++
(which you can indeed link statically) or GCC itself.
You could use an alternative Libc, such as MUSL libc (which is supposed to be more friendly with static linking)
Also, there might be some kernel dependencies.
Normally it can't. It will complain about libc being too old, for one.
If you statically link with libstdc++ and carefully avoid newer glibc features, you may be able to get away with it. The latter is not always possible though. Static linking with libc is not officially supported and may work or not work for you.