From: "Vicente Botet Escriba" <vicente.botet@wanadoo.fr> ...
Do we need to give an initial value just for conversion to work at all, or is it only needed as a return in the error case?
Tony
It is needed for both usages: the initial value when the class is not default constructible, and the error value when we want this value to be returned when failure. Now we can not have an exception for the conversion to a class that is not default constructible because when we give a default value it is also interpreted as the error value. As I said we are mixing both concepts, which is not good, as both are orthogonal.
The is only *one* value provided -- a conversion-failure value. It happens to be used internally for some initialization (for std::stream-based conversions). It's an internal implementation matter and not guaranteed to be that way. What do we achieve by stressing it out? V.