On 26/04/12 23:08, Peter Dimov wrote:
Mathias Gaunard wrote:
On 26/04/12 22:08, Jeffrey Lee Hellrung, Jr. wrote:
Doesn't the same thing happen in boost::optional and boost::variant?
Yes, their code is arguably ill-formed, but I'm sure some people claim otherwise.
No, it's not. Provide the standard text that, in your opinion, makes them ill-formed.
3.10/10. If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined: — the dynamic type of the object, — a cv-qualified version of the dynamic type of the object, — a type similar (as defined in 4.4) to the dynamic type of the object, — a type that is the signed or unsigned type corresponding to the dynamic type of the object, — a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object, — an aggregate or union type that includes one of the aforementioned types among its elements or non- static data members (including, recursively, an element or non-static data member of a subaggregate or contained union), — a type that is a (possibly cv-qualified) base class type of the dynamic type of the object, — a char or unsigned char type. I suppose you probably want to say that the type you're accessing the object with is actually the dynamic type of the object, but that idea is not supported anywhere in the standard. The only mechanism that supports the necessary logic for optional and variant are the new unrestricted unions, where explicit destructor and placement new calls effectively change the active member of the union.