Pretty well I must say, modern C++ is very portable across different compilers. I wrote a quite complex piece of code two months ago, I fully tested it under GCC and Clang, and it built under MSVC without having to change almost anything. It's quite worse under C though, given that the standard way less comprehensive. You really have to stay away from those pesky Glibc extensions, and lots of stuff in the Standard C library is pretty unsafe or straight out broken (like strcpy and friends).