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From: Seungbeom Kim on 28 Jul 2010 05:21
On 2010-07-27 07:45, Daniel Kr�gler wrote:
> On 27 Jul., 07:31, Dragan Milenkovic <dra...(a)plusplus.rs> wrote:
>>
>> And about treating Foo<Derived> as Foo<Base>; it can
>> somewhat be solved by a conversion operator, right?
>
> Typical implementations do this by providing a converting
> constructor, yes. This is the way unique_ptr and shared_ptr
> are specified. This does not allow for binding a *pointer*
> or reference of type unique_ptr<Derived> to a corresponding
> object of type unique_ptr<Base>, though.

Another disadvantage is that, as far as I know, conversion from
unique_ptr<Derived> to unique_ptr<Base> is not considered covariant.

// okay
Base* Base::clone() const;
Derived* Derived::clone() const;

// error
unique_ptr<Base> Base::clone() const;
unique_ptr<Derived> Derived::clone() const;

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Seungbeom Kim

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From: Thiago A. on 28 Jul 2010 10:27

> > * Better error messages
>
> This is in my view the best and only argument for unique type modifier.


One more point:

Sometimes we use pointers only to hold polymorphic items.

For instance, at the beginning the code was like this:

DoSomething(const FilterBase& ) {...}
{
const SpecialFilter specialFilter;
DoSomething(specialFilter);
}
....and then it had to be changed to select the Filter in runtime:

{
std::auto_ptr<const FilterBase> pFilter =
SelectFilterClass(arguments);
DoSomething(*pFilter.get());
}

In this case the pointer is not what I want because I don�t need
the address concept or pFilter++ or iterator concept and it cannot be
null.

All I want is a polymorphic object in this scope.

The point is:

Is the unique modifier applicable for references as well?
What is the "smart reference class"?


If we had this, the code would be expressed as:

{
const unique FilterBase & filter = SelectFilterClass(arguments);
}

Different syntaxes could be:

auto FilterBase & filter = SelectFilterClass(arguments);
delete FilterBase & filter = SelectFilterClass(arguments);

and if could express the idea of
"list of polimorphic items non null"

vector<unique FilterBase& > v;

Everthing is about RAII and the language.


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