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static_cast, cast operator, and constructors.

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  static_cast, cast operator, and constructors. Noah Roberts <dont@email.me> - 2011-07-18 15:46 -0700
    Re: static_cast, cast operator, and constructors. Ian Collins <ian-news@hotmail.com> - 2011-07-19 11:29 +1200
      Re: static_cast, cast operator, and constructors. Noah Roberts <dont@email.me> - 2011-07-18 16:44 -0700
        Re: static_cast, cast operator, and constructors. Ian Collins <ian-news@hotmail.com> - 2011-07-19 11:57 +1200
          Re: static_cast, cast operator, and constructors. Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-07-19 13:12 +0000
            Re: static_cast, cast operator, and constructors. Noah Roberts <dont@email.me> - 2011-07-19 09:26 -0700
              Re: static_cast, cast operator, and constructors. Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-07-20 18:52 +0000
    Re: static_cast, cast operator, and constructors. James Kanze <james.kanze@gmail.com> - 2011-07-20 12:53 -0700

#7945 — static_cast, cast operator, and constructors.

Considder:

struct ObjectA
{
   ObjectA(ObjectA const&);

   template < typename T >
   ObjectA(T const&);

   void fun() const;
};

struct ObjectB
{
   operator ObjectA () const;
};

static_cast<ObjectA>(an_object_b).fun();

Which constructor does the standard specify will be called during the 
static_cast: copy or template?

Behavior I am seeing is that the templated version is called and then 
bases are called on the result of the cast operator.

Is there a way to explicitly call the cast operator or is that what I'm 
supposedly doing?

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#7946

On 07/19/11 10:46 AM, Noah Roberts wrote:
> Considder:
>
> struct ObjectA
> {
>     ObjectA(ObjectA const&);
>
>     template<  typename T>
>     ObjectA(T const&);
>
>     void fun() const;
> };
>
> struct ObjectB
> {
>     operator ObjectA () const;
> };
>
> static_cast<ObjectA>(an_object_b).fun();
>
> Which constructor does the standard specify will be called during the
> static_cast: copy or template?

Is the code complete?  The only option is the template.

> Behavior I am seeing is that the templated version is called and then
> bases are called on the result of the cast operator.
>
> Is there a way to explicitly call the cast operator or is that what I'm
> supposedly doing?

Specialise the template constructor for ObjectB.

template <>
ObjectA::ObjectA<ObjectB>(ObjectB const&)
{
  std::cout << "special" << std::endl;
  }

-- 
Ian Collins

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#7947

On 7/18/2011 4:29 PM, Ian Collins wrote:
> On 07/19/11 10:46 AM, Noah Roberts wrote:
>> Considder:
>>
>> struct ObjectA
>> {
>> ObjectA(ObjectA const&);
>>
>> template< typename T>
>> ObjectA(T const&);
>>
>> void fun() const;
>> };
>>
>> struct ObjectB
>> {
>> operator ObjectA () const;
>> };
>>
>> static_cast<ObjectA>(an_object_b).fun();
>>
>> Which constructor does the standard specify will be called during the
>> static_cast: copy or template?
>
> Is the code complete? The only option is the template.

No, there's a second option: apply the cast operator and call the copy 
operator with the result.

>
>> Behavior I am seeing is that the templated version is called and then
>> bases are called on the result of the cast operator.
>>
>> Is there a way to explicitly call the cast operator or is that what I'm
>> supposedly doing?
>
> Specialise the template constructor for ObjectB.
>
> template <>
> ObjectA::ObjectA<ObjectB>(ObjectB const&)
> {
> std::cout << "special" << std::endl;
> }
>

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#7949

On 07/19/11 11:44 AM, Noah Roberts wrote:
> On 7/18/2011 4:29 PM, Ian Collins wrote:
>> On 07/19/11 10:46 AM, Noah Roberts wrote:
>>> Considder:
>>>
>>> struct ObjectA
>>> {
>>> ObjectA(ObjectA const&);
>>>
>>> template<  typename T>
>>> ObjectA(T const&);
>>>
>>> void fun() const;
>>> };
>>>
>>> struct ObjectB
>>> {
>>> operator ObjectA () const;
>>> };
>>>
>>> static_cast<ObjectA>(an_object_b).fun();
>>>
>>> Which constructor does the standard specify will be called during the
>>> static_cast: copy or template?
>>
>> Is the code complete? The only option is the template.
>
> No, there's a second option: apply the cast operator and call the copy
> operator with the result.

Your terminology is a little confused.  There is a conversion (not cast) 
operator and a copy constructor (not operator).

The sequence you describe required two levels of conversion, the 
template constructor one.

Assignment or omitting the template constructor should call the 
conversion operator:

ObjectA a = an_object_b;

-- 
Ian Collins

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#7967

On Mon, 2011-07-18, Ian Collins wrote:
> On 07/19/11 11:44 AM, Noah Roberts wrote:
>> On 7/18/2011 4:29 PM, Ian Collins wrote:
>>> On 07/19/11 10:46 AM, Noah Roberts wrote:
>>>> Considder:
>>>>
>>>> struct ObjectA
>>>> {
...

>> No, there's a second option: apply the cast operator and call the copy
>> operator with the result.
>
> Your terminology is a little confused.  There is a conversion (not cast) 
> operator and a copy constructor (not operator).

And I personally get confused by a class called "ObjectA". It's a
class, not an object.

/Jorgen

-- 
  // Jorgen Grahn <grahn@  Oo  o.   .     .
\X/     snipabacken.se>   O  o   .

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#7976

On 7/19/2011 6:12 AM, Jorgen Grahn wrote:
> On Mon, 2011-07-18, Ian Collins wrote:
>> On 07/19/11 11:44 AM, Noah Roberts wrote:
>>> On 7/18/2011 4:29 PM, Ian Collins wrote:
>>>> On 07/19/11 10:46 AM, Noah Roberts wrote:
>>>>> Considder:
>>>>>
>>>>> struct ObjectA
>>>>> {
> ...
>
>>> No, there's a second option: apply the cast operator and call the copy
>>> operator with the result.
>>
>> Your terminology is a little confused.  There is a conversion (not cast)
>> operator and a copy constructor (not operator).
>
> And I personally get confused by a class called "ObjectA". It's a
> class, not an object.

You guys get worked up over some pretty weird shit.

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#8003

On Tue, 2011-07-19, Noah Roberts wrote:
> On 7/19/2011 6:12 AM, Jorgen Grahn wrote:
>> On Mon, 2011-07-18, Ian Collins wrote:
>>> On 07/19/11 11:44 AM, Noah Roberts wrote:
>>>> On 7/18/2011 4:29 PM, Ian Collins wrote:
>>>>> On 07/19/11 10:46 AM, Noah Roberts wrote:
>>>>>> Considder:
>>>>>>
>>>>>> struct ObjectA
>>>>>> {
>> ...
>>
>>>> No, there's a second option: apply the cast operator and call the copy
>>>> operator with the result.
>>>
>>> Your terminology is a little confused.  There is a conversion (not cast)
>>> operator and a copy constructor (not operator).
>>
>> And I personally get confused by a class called "ObjectA". It's a
>> class, not an object.
>
> You guys get worked up over some pretty weird shit.

I wasn't worked up. But surely you want your postings to be as clear as
possible?

/Jorgen

-- 
  // Jorgen Grahn <grahn@  Oo  o.   .     .
\X/     snipabacken.se>   O  o   .

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#8004

On Jul 18, 11:46 pm, Noah Roberts <d...@email.me> wrote:
> Considder:

> struct ObjectA
> {
>    ObjectA(ObjectA const&);

>    template < typename T >
>    ObjectA(T const&);

>    void fun() const;
> };
>
> struct ObjectB
> {
>    operator ObjectA () const;
> };

> static_cast<ObjectA>(an_object_b).fun();

> Which constructor does the standard specify will be called during the
> static_cast: copy or template?

My first reaction is that it's ambiguous.  In both cases
(supposing "an_object_b" has type ObjectB), you need a const
conversion, followed by a user defined conversion.  There is
a rule that all other things being equal, a non-template will
have precedence over a template, but I'm not sure it applies
here.  And there are all sorts of subtilities involving overload
resolution in the presence of const; I don't have a copy of the
standard here to try to work them out.

> Behavior I am seeing is that the templated version is called and then
> bases are called on the result of the cast operator.

> Is there a way to explicitly call the cast operator or is that what I'm
> supposedly doing?

There's no way of explicitly calling either the conversion
operator or the constructor; the only way either gets called is
because of a conversion (explicit or implicit).  My feeling is
that either the conversion is ambiguous, or perhaps the
conversion operator should be called, because of the
non-template over template rule; but without having a copy of
the standard to verify it by, I wouldn't want to swear on it.

--
James Kanze

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