Fernando Cacciola wrote:
Hamish Mackenzie wrote:
On Mon, 2005-10-17 at 16:09 -0300, Fernando Cacciola wrote:
But that's not because optional<> is a pointer, is becasue both optional<> and pointer's are somehing else: OptionalPointees.
Let me say first I don't use optional references (yet anyway) and I am quite happy with your explanation of the need to rebind on assignment.
Good.
I am, however, still not convinced that OptionalPointees is justification for using the interface of a pointer.
OK
I still sometimes think of optional as a vector with a maximum size of one, but I now think it would be a bad idea to copy member names from std::vector, as it is likely to cause confusion when using optional< vector< T > >. For the same reason I think optional< bool > is very problematic with the current interface (see my earlier post in this thread).
The problems with optional<bool> are totally unrelated to the choice of operators * and ->. It is the safe_bool() operator _alone_ which causes that. We can of course discuss whether such operator is worth it given the ambiguity it creates with optional<bool>. That was discussed at length at the time optional<> was reviewed (with me initially opposing safe_bool()), in the end, we agreed that it had more benefits than this simple counter-example.
Could the unifying concept OptionalPointee be implemented as free functions?
Of course.
Notice that there are alternatives for safe_bool() that doesn't require dropping * and ->. These alternatives fix the problem with optional<bool>. One example is to simply provide operator !, used like this:
if ( !!opt ) is_initialized();
(this is what my initial boost submission did)
wouldn't requiring nil/none comparison make more sense? optional<bool> opt = ...; if(opt != none) { // is initialized if(opt) { // is true ...; } if(!opt) { // is false ...; } } if(opt == none) { // isn't initialized } Would the single conversion rule apply to: optional<char> oc = next_char(); if(oc == none) // input failed! recover(); if(oc) // optional<char> -> char -> bool ? // not the end of input print(oc);