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digitalmars.D.learn - Using out qualifier instead of *, how to pass null?

reply Rick Mann <rmann-d-lang latencyzero.com> writes:
In wrapping this C API, I've been going back and forth between converting
"Foo*" output parameters to "out Foo". I prefer the latter, as I don't have to
take the address at the call site, but I don't seem to be able to pass null
(which the API allows, meaning, "I'm not interested in this return value").

Am I just stuck with Foo*? Alternatively, I can overload the C function with D
wrappers, I suppose. Hmm. That's probably best.
Feb 01 2007
next sibling parent reply Kirk McDonald <kirklin.mcdonald gmail.com> writes:
Rick Mann wrote:

 In wrapping this C API, I've been going back and forth between converting
"Foo*" output parameters to "out Foo". I prefer the latter, as I don't have to
take the address at the call site, but I don't seem to be able to pass null
(which the API allows, meaning, "I'm not interested in this return value").
 
 Am I just stuck with Foo*? Alternatively, I can overload the C function with D
wrappers, I suppose. Hmm. That's probably best.


You can't pass it null, for the same reason that you can't pass null to 
a C++ function with a reference parameter.

I would recommend binding the C API as closely as you can, and then 
using D's features in a wrapper around it. Generally speaking, this will 
be the most robust and portable approach. It will give you the 
convenient D version for most uses, and the raw C version when you need 
that.

-- 
Kirk McDonald
Pyd: Wrapping Python with D
http://pyd.dsource.org
Feb 01 2007
parent =?ISO-8859-1?Q?Anders_F_Bj=F6rklund?= <afb algonet.se> writes:
Kirk McDonald wrote:


 I would recommend binding the C API as closely as you can, and then 
 using D's features in a wrapper around it. Generally speaking, this will 
 be the most robust and portable approach. It will give you the 
 convenient D version for most uses, and the raw C version when you need 
 that.


Having just rewritten parts of wxD, I second recommending that approach.
(for wxWidgets it had to be done anyway, because it's a C++ API library,
but generally speaking it is easier to keep each language to its own...)
This normally also includes offering legacy aliases, for C portability.

--anders

PS. Rick, for your API wrapper you might find this article interesting:
http://www.eclipse.org/articles/Article-SWT-Design-1/SWT-Design-1.html
Bonus points are awarded for completing the Mac OS X DWT port to Carbon,
that Carlos started on: http://dsource.org/forums/viewtopic.php?t=1169
Feb 01 2007
prev sibling parent reply Derek Parnell <derek psych.ward> writes:
On Thu, 01 Feb 2007 03:00:01 -0500, Rick Mann wrote:


 In wrapping this C API, I've been going back and forth
 between converting "Foo*" output parameters to "out Foo". 
 I prefer the latter, as I don't have to take the address
 at the call site, but I don't seem to be able to pass null
 (which the API allows, meaning, "I'm not interested in
 this return value").
 
 Am I just stuck with Foo*? Alternatively, I can overload
 the C function with D wrappers, I suppose. Hmm. That's
 probably best.


Is not possible to use a 'null' object ... ?


class Foo {}
void Bar(out Foo f) 
{ 
  if (f is null)
      ...  
  else
      ...
}
Foo NullFoo;
Foo RealFoo = new Foo;

Bar(NullFoo);
Bar(RealFoo);

-- 
Derek Parnell
Feb 01 2007
next sibling parent =?ISO-8859-1?Q?Anders_F_Bj=F6rklund?= <afb algonet.se> writes:
Derek Parnell wrote:


Am I just stuck with Foo*? Alternatively, I can overload
the C function with D wrappers, I suppose. Hmm. That's
probably best.


 
 Is not possible to use a 'null' object ... ?


Probably not to extern C, without a D wrapper ?

--anders
Feb 01 2007
prev sibling parent reply xs0 <xs0 xs0.com> writes:
Derek Parnell wrote:

 On Thu, 01 Feb 2007 03:00:01 -0500, Rick Mann wrote:
 

 In wrapping this C API, I've been going back and forth
 between converting "Foo*" output parameters to "out Foo". 
 I prefer the latter, as I don't have to take the address
 at the call site, but I don't seem to be able to pass null
 (which the API allows, meaning, "I'm not interested in
 this return value").

 Am I just stuck with Foo*? Alternatively, I can overload
 the C function with D wrappers, I suppose. Hmm. That's
 probably best.


 
 Is not possible to use a 'null' object ... ?
 
 
 class Foo {}
 void Bar(out Foo f) 
 { 
   if (f is null)
       ...  
   else
       ...
 }
 Foo NullFoo;
 Foo RealFoo = new Foo;
 
 Bar(NullFoo);
 Bar(RealFoo);


hmm.. doesn't out cause (f is null) to be always true in that code?


xs0
Feb 01 2007
parent Derek Parnell <derek nomail.afraid.org> writes:
On Fri, 02 Feb 2007 00:38:30 +0100, xs0 wrote:


 Derek Parnell wrote:

 On Thu, 01 Feb 2007 03:00:01 -0500, Rick Mann wrote:
 

 In wrapping this C API, I've been going back and forth
 between converting "Foo*" output parameters to "out Foo". 
 I prefer the latter, as I don't have to take the address
 at the call site, but I don't seem to be able to pass null
 (which the API allows, meaning, "I'm not interested in
 this return value").

 Am I just stuck with Foo*? Alternatively, I can overload
 the C function with D wrappers, I suppose. Hmm. That's
 probably best.


 
 Is not possible to use a 'null' object ... ?
 
 class Foo {}
 void Bar(out Foo f) 
 { 
   if (f is null)
       ...  
   else
       ...
 }
 Foo NullFoo;
 Foo RealFoo = new Foo;
 
 Bar(NullFoo);
 Bar(RealFoo);


 
 hmm.. doesn't out cause (f is null) to be always true in that code?


LOL... yes it does. I was concentrating on the problem - "I don't seem to
be able to pass null", but missed that obvious gotcha. This code below
actually works though...

// ------------
import std.stdio;

class Foo
{
    char[] m_id;

    this(char[] x) { m_id = x.dup; }
}
void Bar(inout Foo f)
{
  if (f is null)
      writefln("NULL");
  else
      writefln("Got %s", f.m_id);
}
void main()
{
    Foo NullFoo;
    Foo FooA = new Foo("Fred");
    Foo FooB = new Foo("Betty");

    Bar(NullFoo);
    Bar(FooB);
    Bar(FooA);
}
// ------------

-- 
Derek
(skype: derek.j.parnell)
Melbourne, Australia
"Down with mediocrity!"
2/02/2007 11:04:56 AM
Feb 01 2007