Thread: "Operator must be a member function..." (Error)

Why is this invalid? Why does this have to be a member function to a class, and not a stand-alone function like I want?
(String is a class I've made)

Code:

String operator +(const char* TempString1, const char* TempString2);

Info :Compiling C:\Dokument\Cpp\Projekt\String\Test.cpp
Error: STRING.H(80,69):'operator +(const char *,const char *)' must be a member function or have a parameter of class type