Undefined Symbols for Linker on only one definition

Here is another "undefined symbols" problem to add to the plethora of undefined symbols questions on Stack Overflow. Most of my experience is in C, so this is most likely a C++ issue, but it may not be.

I have a header file with the following definitions:

namespace abc {

class Sim {
    public:
        int mytest();
        Regex *RegexComp(std::vector<std::string>, int, int);
}
}

where Regex is a struct defined elsewhere in the file. I have the implementations in a C file:

Regex *Sim::RegexComp(std::vector<std::string> patterns, int type. int opt) {
    // blah blah blah
}

int Sim::mytest() {
    cout << "Test\n";
}

I compile this into a shared library (OS X), and if I run nm on it, it shows that the RegexComp is being export

shell> nm -g bin/libregex.dylib | c++flit
       ...
       ...
       0000000000001740 T abc::Sim::RegexComp(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >, int, int)

However, when I attempt to use this library in another file,

mytest()

can be called successfully but RegexComp results in

Undefined symbols for architecture x86_64:
  "udpregex::UdpSim::RegexComp(std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, int, int)", referenced from:
      _main in ccP5qbaP.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

What is going on here?