Aggregates & associations

[Sean Kelly]

Steven Schveighoffer wrote:
> "Sean Kelly" wrote
>> A long time ago Walter mentioned that the "scope" attribute may be extended 
>> to indicate aggregation:
>>
>>     class Foo
>>     {
>>         Bar valA;
>>         scope Baz val2;
>>     }
>>
>> With the above, the size of Foo would be expanded to contain an instance 
>> of Baz and val2 would be initialized to reference this memory, very much 
>> like "scope" works in functions now.  But I didn't see any mention of this 
>> in the D 2.0 writeup so the idea may have been either discarded or 
>> forgotten.
> 
> I believe this type of behavior is not necessary.  Whether a member is an 
> aggregate or an association, it will be deleted when nothing references it 
> anymore, which is when it should be deleted.  The only thing this proposed 
> behavior can do is cause segfaults when something that still has a reference 
> to a scoped member tries to use it.

You're right that it's not necessary.  The GC is supposed to take care 
of this for you.  But constructing complex aggregates can be quite 
expensive in terms of GC use, as a separate allocation must be made for 
every object.  This can also reduce locality of the objects are of 
different sizes, which may affect performance by incurring cache misses. 
  Now, it is possible in D 2.0 to get the size of an object and reserve 
space for it using a static array, but this requires the use of 
placement new, which requires yet more fancy template code to bolt onto 
existing classes, etc.  So this is certainly possible, but far messier 
than in C++.  And it's really a fairly common design requirement.


Sean