System::String for basic_string

Question

I was looking for how to convert System::String to "normal" string, and found a command:

#include <iostream>
#include <string>
#include <msclr\marshal_cppstd.h>
System::String^ mstr = "PTSO";
std::string mstrr msclr::interop::marshal_as< string >(mstr);

It worked, but I tried to make a function to "facilitate" the use, and I could not. I tried so:

#include <iostream>
#include <string>
#include <msclr\marshal_cppstd.h>
string SysStrToStr(System::String^ sstr) {
        return msclr::interop::marshal_as< std::string >(sstr);
    }

Made that mistake:

1>Source.obj : error LNK2005: "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __clrcall SysStrToStr(class System::String ^)" (?SysStrToStr@@YM?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@P$AAVString@System@@@Z) already defined in MyForm.obj

Clarifying: I’m creating a CLR(Forms) application in Visual Studio 2012 (C++), and when I need to pass the contents of a Richtextbox(text) to a function, it sends System::string, and I always need to convert to "normal" string (Std::string) to use.