Destructor semantics

[Steven Schveighoffer]

On Tue, 10 Aug 2010 17:23:39 -0400, foobar <foo at bar.com> wrote:

> Steven Schveighoffer Wrote:
>
>> On Tue, 10 Aug 2010 16:33:08 -0400, foo <foo at bar.com> wrote:
>>
>> > In light on recent discussions of clear() and the distructor it seems  
>> to
>> > me that we are going backwards from one of D's great improvements over
>> > C++ - the difference in semantics between structs and classes.
>> >
>> > IMO, instead of enhancing class desteructors they should be completely
>> > removed and only allowed on structs with deterministic semantics and  
>> all
>> > uses cases of class desteructors should be replaced with structs.
>> > Examples:
>> > class SocketConnection : Connection {
>> > // struct instance allocated inline
>> > SocketHandle handle;
>> > ...
>> > }
>> >
>> > OR:
>> >
>> > class SocketConnection : Connection {
>> > struct {
>> >    this()  { acquireHandle(); }
>> >   ~this() { releaseHandle(); }
>> > } handle;
>> > ...
>> > }
>> >
>> > The suggested semantics of the above code would be that creating a
>> > SocketConnection object would also construct a SocketHandle as part of
>> > the object's memory and in turn that would call the struct's ctor.
>> > On destruction of the object, the struct member would be also  
>> destructed
>> > and it's d-tor is called. This is safe since the struct is part of the
>> > same memory as the object.
>> >
>> > in short, struct instances should be treated just like built-in types.
>> >
>>
>> That doesn't help.  deterministic destruction is not a struct-vs-class
>> problem, its a GC-vs-manual-memory problem.  A struct on the heap that  
>> is
>> finalized by the GC has the same issues as a class destructor.  In fact,
>> struct destructors are not currently called when they are heap-allocated
>> because the GC has no idea what is stored in those memory locations.
>>
>> -Steve
>
> Let me add to the above, that the GC should NOT manage structs allocated  
> on the heap. structs should only provide deterministic semantics.

So either you are saying that structs that are in classes are never  
destroyed, and you have a resource leak, or every class has an  
auto-generated destructor that calls the struct destructors, and we have  
the same determinism problem you purport to solve.  If a struct is in a  
class, it's on the heap.  You have not solved the problem.

-Steve