On 5 Oct 2010, at 15:11, Sebastian Redl wrote:
On 05.10.2010 15:54, Krzysztof Czainski wrote:
2010/10/5 John Reid<j.reid@mail.cryst.bbk.ac.uk>
Rutger ter Borg wrote:
Hello,
I'm looking into boost::optional<> as a replacement for some of my code which has to deal with optional values. However, it seems that cases which are usually space-efficient optionals (e.g., ptrs in the form of a null-ptr) do not have template specializations. In other words, boost optional templates store a bool and the memory footprint of its template parameter, regardless of type. I.e., optional references and pointers take their space plus the space of an extra bool.
Is there a reason for this?
Might someone want to store a null pointer in the optional? I'm not sure
about the references though.
John.
I understand, that a null pointer means the same as lack of a pointer.
In your use case.
The idea has been discussed on the list before, and IIRC, the conclusion was that the library simply cannot rely on the user not wanting to have a null pointer in an optional. I could see a trait for optional specifying if a type has a natural empty value, and if so, which.
Yes, for example while NULL is a sensible pointer to store, I decided I would never want to represent (void*)1, so I specialised boost::optional to use that. It is also possible to come up with impossible states for most standard library objects (std::vector, std::list) if you know how they are stored internally. However I'm not sure such code could be submitted to boost.
Sebastian _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost