Re: [boost] New Library Proposal: dual_state

From: David Abrahams (dave_at_[hidden])
Date: 2005-07-16 08:43:31

"Calum Grant" <calum_at_[hidden]> writes:

>> > I've been following this dual_state vs optional discussion for a 
>> > while. It seems to me that one should really have a policy 
>> class that 
>> > defines the default behaviour when the value is unassigned.  This 
>> > policy should define whether to
>> >
>> >   - throw an exception
>> >   - construct an object on the fly
>> >   - return a default 
>> >   - return a null pointer
>> >   - whether to return by value or by reference
>> >
>> > A second point, can't all this be achieved by adding 
>> policies to smart 
>> > pointers?
>> 
>> Whoa; smart pointers and boost::optional are not very closely 
>> related beasts.  
>
> I mean, a weak_ptr and optional both have 0 or 1 pointees, so there is
> at least a passing resemblance.  

Merely a syntactic one.  optional is not a smart pointer; it just uses
* and -> to provide access to its contained object.

> In the absence of Boost, I would probably use a std::auto_ptr to
> store an optional field, which is, um, a smart pointer.  I realize
> that Optional is implemented slightly differently.

I wouldn't characterize it as a "slight" difference.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com