It is illegal to use forward declarations with typedef names, as in:

    class string;   void f(string& s); //illegal, string is a typedef name

Even a typename won’t do here:

    typename  std::string; //still illegal   void f(std::string& s);  

The problem with these forward declarations is that std::string is not a class, but a typedef name defined like this:

    typedef basic_string, allocator >string;

In order to generate the correct mangled name for the function f, the compiler has to see the non-typedef’d form of its argument. In other words, there is no escape from #including the header in this case.