Re: [boost] [unnullable_ptr] Proposal for a new library

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-07-24 12:23:05

AMDG

Giovanni Piero Deretta wrote:
> On Thu, Jul 24, 2008 at 4:58 PM, Mathias Gaunard
> <mathias.gaunard_at_[hidden]> wrote:
> <snip>
>   
>> T::T(T&& o) could make 'o' empty, but there could be no other way to
>> make 'o' empty otherwise.
>>     
>
> I think that the biggest problem is that, if T is movable,  you cannot
> statically guarantee that T is never accessed in an empty state, so
> the never empty guarantee is not as useful.
>   

Upon further reflection, the never empty guarantee is not
useful at all for Boost.Function.

Suppose that Boost.Function does not provide such a guarantee
and I have a boost::function that I want to call.  What can I assume?
I obviously can't assume that it won't throw, because it will throw
if it is empty.  This is not changed by adding the never empty guarantee.
Nothing prevents a boost::function from holding a function object
that throws when it is called.

The only invariant that is added by the never empty guarantee is
that f.target_type() != typeid(void).  I don't see this as being useful.

In Christ,
Steven Watanabe