C++, PluginMain() and file types

I'm attempting to write a little do-nothing plugin, and not having it compile.  This is on a Mac, 64-bit Snow Leopard, using XCode and the CS5 SDK.

At first I tried having the main source code file (the only source code file) be .cpp, and it didn't compile.  Searching forums and Google, Someone said there's a filetype to change the file to, objective-c++, sourcecode.cpp.objccpp (sp?).   I tried it.    It still didn't compile, but now for other reasons.  Now I look in the Dissolve example, clicking on Get Info for the file with PluginMain() and see it is the plain vanilla C++ file type, sourcecode.cpp.cpp.   Yet Dissolve *does* compile!    What is the explanation?