Objective C with some C++ code won't compile

I have an Objective C project with some C++ code, and sure enough the C++ code won't compile

I understand that the compiler makes assumptions about which language to compile, but I have seen this app running before so I am curious how to make my Xcode 4 run compile this project

ie. This function declaration produces an error as the compiler does not recognize the c++std:: syntax

 std::string string_with_formatting(const unsigned int value) {

How do I get around this? What #include or #import should I have