On 05/10/2010 11:11 a.m., 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.
Exactly. optional<> cannot decide *by itslef* that a particular value happens to be equivalent to an uninitialized state. That is, a null pointer is not neccesarily the same as none. This is particulary true in a generic design when the type wrapped is unconstrained (i.e. T can be anything, including a pointer) and the condition of uninitialized state *must* be strict (i.e. not *any* value being given)
I could see a trait for optional specifying if a type has a natural empty value, and if so, which.
This and other desirable policies has also been discussed, but the general idea of a policy-based design was rejected for this utility. There where lengthly discussions about such designs for "simple wrappers" like shared_ptr<> or optional<> several years ago. The simple rational being that it clutters the compile-time interface. -- Fernando Cacciola SciSoft Consulting, Founder http://www.scisoft-consulting.com