Class members should not be const references to string

It is unsafe to have const string reference members as they can be created from a temporary quite easily.

struct Foo {
  Foo(const string &Str) : Str(Str) {}
  const string &Str;
};
Foo instance("string");

When the constructor is called, a string temporary is created from const char * and destroyed immediately after. This results in a dangling reference.

This rule applies for both std::string and ::string const reference members.