Hi everyone,

Was hoping someone could help me make sense of this bit of C++ code:

class canvas
{
    operator HDC() { return _hdc; }
protected:
    canvas(HDC hdc): _hdc(hdc) {}
    HDC _hdc;
}

From what I understand, HDC is an alias for HANDLE in Windows. So they are overloading canvas such that when assigned to handle instance it returns _hdc? How is this done in D?

Thanks

On 23.02.2011 17:08, %u wrote:
> Hi everyone,
>
> Was hoping someone could help me make sense of this bit of C++ code:
>
> class canvas
> {
>      operator HDC() { return _hdc; }
> protected:
>      canvas(HDC hdc): _hdc(hdc) {}
>      HDC _hdc;
> }
>
>  From what I understand, HDC is an alias for HANDLE in Windows. So
> they are overloading canvas such that when assigned to handle
> instance it returns _hdc? How is this done in D?
>
> Thanks

Something like this, and i shouldn't be a class in D  (it's a proxy sort of, right? )  :

struct canvas{
   alias getHandle this;
   this(HDC hdc){
     _hdc = hdc;
   }
   @property  HDC getHandle(){ return _hdc; }
private:
    HDC _hdc;
}

uinttest{
    HDC h = cast(HDC)1;
    canvas c = h;//intitalize
    h = c;
    assert(h == c);
}

-- 
Dmitry Olshansky

On 23/02/2011 14:37, Dmitry Olshansky wrote:
> On 23.02.2011 17:08, %u wrote:
>> Hi everyone,
>>
>> Was hoping someone could help me make sense of this bit of C++ code:
>>
>> class canvas
>> {
>> operator HDC() { return _hdc; }
>> protected:
>> canvas(HDC hdc): _hdc(hdc) {}
>> HDC _hdc;
>> }
>>
>> From what I understand, HDC is an alias for HANDLE in Windows. So
>> they are overloading canvas such that when assigned to handle
>> instance it returns _hdc? How is this done in D?
>>
>> Thanks
>
> Something like this, and i shouldn't be a class in D (it's a proxy sort
> of, right? ) :
>
> struct canvas{
> alias getHandle this;
> this(HDC hdc){
> _hdc = hdc;
> }
> @property HDC getHandle(){ return _hdc; }
> private:
> HDC _hdc;
> }
>
> uinttest{
> HDC h = cast(HDC)1;
> canvas c = h;//intitalize
> h = c;
> assert(h == c);
> }
>

That's the wrong way round.

The c++ class is a canvas object which can be transparently passed to any windows function which takes a HDC.

Note the constructor is protected and the cast operator is public.

Not sure of the cast operator details in D though; I've never used it myself.

-- 
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk

On 23/02/2011 18:42, Simon wrote:
> On 23/02/2011 14:37, Dmitry Olshansky wrote:
>> On 23.02.2011 17:08, %u wrote:
>>> Hi everyone,
>>>
>>> Was hoping someone could help me make sense of this bit of C++ code:
>>>
>>> class canvas
>>> {
>>> operator HDC() { return _hdc; }
>>> protected:
>>> canvas(HDC hdc): _hdc(hdc) {}
>>> HDC _hdc;
>>> }
>>>
>>> From what I understand, HDC is an alias for HANDLE in Windows. So
>>> they are overloading canvas such that when assigned to handle
>>> instance it returns _hdc? How is this done in D?
>>>
>>> Thanks
>>
>> Something like this, and i shouldn't be a class in D (it's a proxy sort
>> of, right? ) :
>>
>> struct canvas{
>> alias getHandle this;
>> this(HDC hdc){
>> _hdc = hdc;
>> }
>> @property HDC getHandle(){ return _hdc; }
>> private:
>> HDC _hdc;
>> }
>>
>> uinttest{
>> HDC h = cast(HDC)1;
>> canvas c = h;//intitalize
>> h = c;
>> assert(h == c);
>> }
>>
>
> That's the wrong way round.

Nuts never mind. I'm talking out my ass.
Though how do you do it using the cast operator?
That would be closer to the c++ implementation.

-- 
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk

On Wednesday, February 23, 2011 10:42:04 Simon wrote:
> On 23/02/2011 14:37, Dmitry Olshansky wrote:
> > On 23.02.2011 17:08, %u wrote:
> >> Hi everyone,
> >> 
> >> Was hoping someone could help me make sense of this bit of C++ code:
> >> 
> >> class canvas
> >> {
> >> operator HDC() { return _hdc; }
> >> protected:
> >> canvas(HDC hdc): _hdc(hdc) {}
> >> HDC _hdc;
> >> }
> >> 
> >> From what I understand, HDC is an alias for HANDLE in Windows. So they are overloading canvas such that when assigned to handle instance it returns _hdc? How is this done in D?
> >> 
> >> Thanks
> > 
> > Something like this, and i shouldn't be a class in D (it's a proxy sort
> > of, right? ) :
> > 
> > struct canvas{
> > alias getHandle this;
> > this(HDC hdc){
> > _hdc = hdc;
> > }
> > @property HDC getHandle(){ return _hdc; }
> > private:
> > HDC _hdc;
> > }
> > 
> > uinttest{
> > HDC h = cast(HDC)1;
> > canvas c = h;//intitalize
> > h = c;
> > assert(h == c);
> > }
> 
> That's the wrong way round.
> 
> The c++ class is a canvas object which can be transparently passed to any windows function which takes a HDC.
> 
> Note the constructor is protected and the cast operator is public.
> 
> Not sure of the cast operator details in D though; I've never used it myself.

You can overload the cast operator in D:

    T opCast(T)() {...}

but it's for explicit cast only. There is not currently any way to do implicit casts with an overloaded operator. You have to use alias this for that, which may or may not do what be what you want if you want an implicit cast.

- Jonathan M Davis

On 23.02.2011 21:48, Simon wrote:
> On 23/02/2011 18:42, Simon wrote:
>> On 23/02/2011 14:37, Dmitry Olshansky wrote:
>>> On 23.02.2011 17:08, %u wrote:
>>>> Hi everyone,
>>>>
>>>> Was hoping someone could help me make sense of this bit of C++ code:
>>>>
>>>> class canvas
>>>> {
>>>> operator HDC() { return _hdc; }
>>>> protected:
>>>> canvas(HDC hdc): _hdc(hdc) {}
>>>> HDC _hdc;
>>>> }
>>>>
>>>> From what I understand, HDC is an alias for HANDLE in Windows. So
>>>> they are overloading canvas such that when assigned to handle
>>>> instance it returns _hdc? How is this done in D?
>>>>
>>>> Thanks
>>>
>>> Something like this, and i shouldn't be a class in D (it's a proxy sort
>>> of, right? ) :
>>>
>>> struct canvas{
>>> alias getHandle this;
>>> this(HDC hdc){
>>> _hdc = hdc;
>>> }
>>> @property HDC getHandle(){ return _hdc; }
>>> private:
>>> HDC _hdc;
>>> }
>>>
>>> uinttest{
>>> HDC h = cast(HDC)1;
>>> canvas c = h;//intitalize
>>> h = c;
>>> assert(h == c);
>>> }
>>>
>>
>> That's the wrong way round.
>
> Nuts never mind. I'm talking out my ass.
> Though how do you do it using the cast operator?
> That would be closer to the c++ implementation.
>
There is no much point in getting closer to the source I think.
Well, there sort of was opImplicitCast, but it's functionality is superseded by alias this.
When multiple alias this are implemented and some bugs fixed we are sure to witness it's full power.

Meanwhile you could just as well make this proxy a template struct:
struct proxy(T){
   alias getHandle this;
   this(T handle){
     _h = handle;
   }
   @property  T getHandle(){ return _h; }
private:
    T _h;
}
alias proxy!(HDC) canvas;
//and so on and so forth

But this sort of thin wrappers are of no good if they do not provide
you some benefits like automatic resource management and such.

-- 
Dmitry Olshansky

On 23/02/2011 19:56, Jonathan M Davis wrote:
>
> You can overload the cast operator in D:
>
>      T opCast(T)() {...}
>
> but it's for explicit cast only. There is not currently any way to do implicit
> casts with an overloaded operator. You have to use alias this for that, which
> may or may not do what be what you want if you want an implicit cast.
>
> - Jonathan M Davis

thanks for the info; and Dmitry.

-- 
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk

Thaks to everyone for your assistance.