including c++ libraries in ios programming

I'm developing an ios app that's very basic and uses objective almost all of the time. However my app needs to deal with big integer numbers (eg: 2^200) and add and multiply them. To achieve that I need to include a c++ library called bigint that allows these operations on huge integers. The problem I have is that when I include the bigint project I get many errors and I thought that this could be because it's c++ and can't mix with objective c. I'm new to this idea and was wondering if there are any steps i need to take to correctly add a c++ library to an objective c project. By the way I'm not using opengl or anything complicated just simple ui and some quartz stuff.

Update: I did everything you guys said, I changed all the extensions to .mm and added the bigint library. My project ran perfectly without errors before doing these things. I get an error when i do this and I get an error even if I don't even add the library. just changing the file extensions to .mm gives me the following error.

This just doesnt make sense since everything ran fine before and I don't have any duplicates in my program. I have no idea why just changing the extensions to .mm could cause this error. Any ideas guys?

You can mix in C++ files, but use a .cpp suffix for them (and .hpp for their corresponding header files). If you want to mix C++ and Obj-C in the same file, you can do that, but give it a .mm suffix.