Mixing C++ and Objective-C code for iPhone apps

This was something that caused a lot of headache for one of our apps. I downloaded this open source engine to help with the physics of our game, but it was pretty much all written in c++. All the c++ source files themselves would compile just fine, but when I tried to #import the headers files into my Objective-C project, I got about 1300 errors! So, I went on a massive Google search to figure out what was wrong. I was pretty sure it had to do with the c++ syntax in the header files. After a long and exhaustive search, I finally came up with the answer. I kept reading that in order to use c++ code in an Obj-C file, the file extension needed to be .mm, but none of my files actually had c++ in them. Then I came across a vague reference to the fact that even those files that just import c++ files need to have the .mm extension. So I tried it, and it worked! Needless to say, this simple solution made me feel like a complete idiot. But now I know, that in order to use c++ files, any Obj-C files that import the c++ header files NEED to have that stupid .mm extension! Anyways, I hope other people can avoid the same frustration I felt over this stupid predicament.