Kevlin Henney <kevlin@curbralan.com> writes:
In message <uk727uuvv.fsf@boost-consulting.com>, David Abrahams <dave@boost-consulting.com> writes
The conversion to boost::any isn't a widening conversion in the usual sense, though, is it? Normally, a widening conversion preserves not only the information but the *interface* of the original data -- you wouldn't consider an implicit conversion from int to string a widening conversion, would you?
Your assumption is not quite correct: most widening conversions are not interface preserving.
So you're saying it's not even a little correct, it sounds like. Can you give some examples of non-interface-preserving widening conversions?
T * to void *, derived * to base *, int (32 bit) to double (64 bit), T * to const T *, etc. Widening conversions are in the direction of the supertype, hence they often have a subset of the interface of source type. The corresponding narrowing of the interface with the widening of the type is not always the case, eg an int to long is effectively interface preserving -- the same set of operators is supported for both types.
I guess I don't understand the whole concept now. T* -> void* sounds like "narrowing", since it loses information. int -> long never loses information, so "widening" makes sense to me. What is the criterion? -- Dave Abrahams Boost Consulting www.boost-consulting.com