LabVIEW RT does not generate C/C++ code. The block diagrams are compiled directly to assembly language at run-time.
If your question was really whether or not you can use C/C++ code under LabVIEW RT, the answer is yes, through the use of a Call Library Function node (DLL). You have to be careful, however that your DLL does not make any Windows-specific function calls that are not supported under the RT OS. Most of the WinAPI is supported under the RT OS, but the only tool that NI would be able to support making such a DLL would be Measurement Studio 6.0 or later.