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Subject: Re: [boost] [optional] memory use for optional refs and ptrs
From: Fernando Cacciola (fernando.cacciola_at_[hidden])
Date: 2010-10-05 11:57:47

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_at_[hidden]>
>>> 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.


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

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