Under the conditions specified in [dcl.init.ref], a reference can be bound directly to a glvalue or class prvalue that is the result of applying a conversion function to an initializer expression. Overload resolution is used to select the conversion function to be invoked. Assuming that “cv1 T” is the underlying type of the reference being initialized, and “cv S” is the type of the initializer expression, with S a class type, the candidate functions are selected as follows:
The conversion functions of S and its base classes are considered, except that for copy-initialization, only the non-explicit conversion functions are considered. Those that are not hidden within S and yield type “lvalue reference to cv2 T2” (when [dcl.init.ref] requires an lvalue result) or “ cv2 T2” or “rvalue reference to cv2 T2” (when [dcl.init.ref] requires an rvalue result), where “cv1 T” is reference-compatible ([dcl.init.ref]) with “cv2 T2”, are candidate functions.
The argument list has one argument, which is the initializer expression. [ Note: This argument will be compared against the implicit object parameter of the conversion functions. — end note ]