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struct.<field>.offsetof is not supported.

struct.html:
# Struct Field Properties
# .offsetof
#       Offset in bytes of field from beginning of struct

test case:
http://svn.kuehne.cn/dstress/run/offsetof_01.d

Thomas


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On Wed, 1 Dec 2004 13:31:31 +0100, Thomas Kuehne <thomas-dloop@kuehne.thisisspam.cn> wrote:
> struct.<field>.offsetof is not supported.
>
> struct.html:
> # Struct Field Properties
> # .offsetof
> #       Offset in bytes of field from beginning of struct
>
> test case:
> http://svn.kuehne.cn/dstress/run/offsetof_01.d

Try this instead:

// $HeadURL$
// $Date$
// $Author$

module dstress.run.offsetof_01;

struct MyStruct{
	int a;
	int b;	
}

int main(){
	assert(MyStruct.a.offsetof >= 0);
	assert(MyStruct.b.offsetof >= 0);
	assert(MyStruct.a.offsetof != MyStruct.b.offsetof);

	return 0;
}

IIRC this property applies to the struct definition *not* the struct instance.

Regan

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Regan Heath schrieb am Thu, 02 Dec 2004 09:54:55 +1300:
> On Wed, 1 Dec 2004 13:31:31 +0100, Thomas Kuehne
>> struct.<field>.offsetof is not supported.
>>
>> struct.html:
>> # Struct Field Properties
>> # .offsetof
>> #       Offset in bytes of field from beginning of struct
>>
>> test case:
>> http://svn.kuehne.cn/dstress/run/offsetof_01.d
>
> Try this instead:
>
> // $HeadURL$
> // $Date$
> // $Author$
>
> module dstress.run.offsetof_01;
>
> struct MyStruct{
> 	int a;
> 	int b;
> }
>
> int main(){
> 	assert(MyStruct.a.offsetof >= 0);
> 	assert(MyStruct.b.offsetof >= 0);
> 	assert(MyStruct.a.offsetof != MyStruct.b.offsetof);
>
> 	return 0;
> }
>
> IIRC this property applies to the struct definition *not* the struct instance.

That's strange. For classes it's the other way round: the property applies only to the instance and not the class definition !?

I can come up with an explanation why - under some uncommon circumstances - the offsetof property can only be determined for instances.

Why should a property that is available for the definition not be available for the instance?

Thomas


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On Wed, 1 Dec 2004 22:09:07 +0100, Thomas Kuehne <thomas-dloop@kuehne.thisisspam.cn> wrote:
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> Regan Heath schrieb am Thu, 02 Dec 2004 09:54:55 +1300:
>> On Wed, 1 Dec 2004 13:31:31 +0100, Thomas Kuehne
>>> struct.<field>.offsetof is not supported.
>>>
>>> struct.html:
>>> # Struct Field Properties
>>> # .offsetof
>>> #       Offset in bytes of field from beginning of struct
>>>
>>> test case:
>>> http://svn.kuehne.cn/dstress/run/offsetof_01.d
>>
>> Try this instead:
>>
>> // $HeadURL$
>> // $Date$
>> // $Author$
>>
>> module dstress.run.offsetof_01;
>>
>> struct MyStruct{
>> 	int a;
>> 	int b;	
>> }
>>
>> int main(){
>> 	assert(MyStruct.a.offsetof >= 0);
>> 	assert(MyStruct.b.offsetof >= 0);
>> 	assert(MyStruct.a.offsetof != MyStruct.b.offsetof);
>>
>> 	return 0;
>> }
>>
>> IIRC this property applies to the struct definition *not* the struct
>> instance.
>
> That's strange. For classes it's the other way round: the property
> applies only to the instance and not the class definition !?
>
> I can come up with an explanation why - under some uncommon
> circumstances - the offsetof property can only be determined for
> instances.

For classes but not for structs. For structs you are supposed to be able to align and lay them out how you want. So at compile time the offsets are known. I suspect that is why the definition is used for structs and the instance for classes.

> Why should a property that is available for the definition not be
> available for the instance?

Good question. If it's available for the definition then surely it can be made available for the instance.. it might be an optimisation thing, why store the info 'in' the struct when it's not required. However surely some compiler trickery could deduce the struct type from the instance and turn..

struct foobar {
  int a;
}

foobar b;
b.a.offsetof


into..

struct foobar {
  int a;
}

foobar b;
foobar.a.offsetof

Regan


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