Re: [Boost-users] Covariant return type, method hiding, and shared_ptr advice

Subject: Re: [Boost-users] Covariant return type, method hiding, and shared_ptr advice
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-04-15 21:35:57

AMDG

Dominique Devienne wrote:
> On Wed, Apr 15, 2009 at 12:25 PM, Steven Watanabe <watanabesj_at_[hidden]> wrote:
>   
>> Dominique Devienne wrote:
>>     
>>> So I used covariant return type, which AFAIK works only for raw
>>> pointers. If I were to remove the raw pointer accessors, I'd have to
>>> do either:
>>>       
>> I would probably use a non-member template that
>> encapsulates the necessary logic:
>>
>> template<class T>
>> shared_ptr<typename T::IdentType> getIdent(T& t);
>>     
>
> I have:
>
> template <class T> shared_ptr<typename T::ident_type> ident_of(T* t) {
>     BOOST_STATIC_ASSERT((is_base_of<Object, T>::value));
>     return static_pointer_cast<typename T::ident_type>(t->ident());
> }
> template <class T> shared_ptr<typename T::ident_type const>
> ident_of(const T* t) {
>     BOOST_STATIC_ASSERT((is_base_of<Object, T>::value));
>     return static_pointer_cast<typename T::ident_type const>(t->ident());
> }
>
> but it chokes on:
>
>     scoped_ptr<Car> z(new Car("nissan", "370Z"));
>     ident_of(z)->set_make("datsun");
>
> error C2784: 'boost::shared_ptr<T::ident_type> ident_of(T *)' :
> could not deduce template argument for 'T *' from 'boost::scoped_ptr<T>'
>
> Instead of doing overloads for various smart pointer types, is it
> possible (and easy) to do the ChainedPtr thing that B.MI does for the
> ident_of template functions above?
>   

Try passing the argument to ident_of by reference instead of by pointer
and dereference the (smart) pointer at the call site.

In Christ,
Steven Watanabe