Re: [boost] [interest] rich-typed smart pointers

Subject: Re: [boost] [interest] rich-typed smart pointers
From: Larry Evans (cppljevans_at_[hidden])
Date: 2013-10-06 08:26:15

On 10/05/13 23:14, Larry Evans wrote:
> On 10/05/13 20:14, Larry Evans wrote:
>> On 10/05/13 16:26, Julian Gonggrijp wrote:
>>> Larry Evans wrote:
>>>
>>>>> https://github.com/jgonggrijp/rich-typed-pointers
>>>> [snip]
>>>> If there were a templated owned_ptr CTOR:
>>>>
>>>>     template<typename Derived>
>>>>     owner_ptr (owner_ptr<Derived> && source) :
>>>> pointer(source.pointer) {
>>>>         source.pointer = nullptr;
>>>>     }
>>>>
>>>> wouldn't that eliminate the need for the make_dynamic templated
>>>> function?
>>>>
>>>> Just wondering.  I haven't tried it, but I'd guess it would work
>>>> because
>>>> the ownd_ptr<Derived> could be constructed with the:
>>>>
>>>>   make<Derived>(t1,t2,...tn)
>>>>
>>>> and then the templated owned_ptr CTOR would then convert that to the
>>>> base class?
>>>
>>> I agree this would be great, but unfortunately it appears it can't be
>>> done.
>>> owner_ptr<Derived> would have to friend-declare owner_ptr<Base>,
>> I don't see why.  AFAICT, you'd require *fewer* friend declarations, not
>> more( you wouldn't need the:
>>
>>      template <class U1, class U2, class ... Us>
>>      friend owner_ptr<U1> make_dynamic (Us&& ...);
>>
>> the only friend needed would the existing:
>>
>>      template <class U, class ... Us>
>>      friend owner_ptr<U> make (Us&& ...);
>>
>> that's because the proposed templated constructor would have the same
>> access as the existing:
>>
>>      owner_ptr (owner_ptr && source);
>>
>> CTOR, i.e. public.  IOW, with the proposed templated CTOR, the
>> following example code creating an owner_ptr<Base> should work,
>> AFAICT:
>>
>>      owner_ptr<Base>
>>    owner_baee
>>      ( make<Derived>
>>        ( DerivedInit0()
>>        , DerivedInit1()
>>        .
>>        .
>>        . DerivedInitN()
>>        )
>>      );
>>
>>
>> Where DeriveInit0, DerivedInit1, ... DerivedInitN
>> are the types of the args to the CTOR for Derived.
>>
>> Is there some reason why the above example code would
>> *not* compile, given then proposed templated CTOR?
>> [snip]
>>
> Coded it but it failed to compile because
> owner_ptr<shape> could not assign a nullptr to
> owner_ptr<circle>::pointer because (as you mentioned)
> owner_ptr<shape> is not a fried of owner_ptr<circle>.
>
> Sorry for noise.
>
Googling:

   c++ templated friend

lead to:

   http://stackoverflow.com/questions/3292795/template-friend

which lead to the owner_ptr CTOR code:

     template<typename Derived>
     owner_ptr (owner_ptr<Derived>&& source) : pointer(source.pointer) {
         source.pointer = nullptr;
     }
     template <class Derived>
     friend class owner_ptr;

in rich_typed_ptr.hpp which resulted in successful compile and run
of the shapes.cpp example without using make_dynamic.

HTH.

-regards,
Larry