deprecated delete and manual memory management

Daniel Gibson metalcaedes at gmail.com
Tue Apr 26 17:15:45 PDT 2011


Am 27.04.2011 02:03, schrieb Steven Schveighoffer:
> On Tue, 26 Apr 2011 19:45:21 -0400, Daniel Gibson
> <metalcaedes at gmail.com> wrote:
> 
>> I just noticed something regarding clear():
>>   struct Bar { ... }
>>   Bar *b = new Bar();
>> This compiles without any warning, but doesn't call the destructor:
>>   clear(b);
>> This compiles without any warning and actually does what you'd expect,
>> i.e. calls destructor:
>>   clear(*b);
>>
>> Is this behavior expected/wanted? If not: Is it a known bug? I couldn't
>> find it in the bugtracker, but maybe I searched for the wrong keywords.
> 
> Let's start with class references.  Because class references cannot be
> separated from its reference, you have to finalize the class when
> finalizing the reference, because there's no way to say "clear what this
> reference refers to" vs. "clear this reference".  So you have to give a
> way to finalize a class instance.
> 
> With pointers, however, you can specify as you say, whether you want to
> clear the pointer or the struct itself.
> 
> Now, is it much useful to clear a pointer without clearing what it
> points to?  I'd say no, clearing a pointer is as easy as doing ptr =
> null.  So I'm thinking, it should be filed as a bug.
> 
> The obvious thing to decide is, what should be done on references to
> references?  If you clear a double pointer, should it go through both
> pointers?  Or a pointer to a class reference?
> 
> I'd say no, but you have to take extra steps to ensure it is this way.
> 
> -Steve

IMHO clear isn't needed for anything but structs and Objects.
For any simple type or pointer you can just write x = x.init; instead of
clear(x) or, as you already mentioned, x=null; for pointers.

AFAIK the main purpose of clear() is to explicitly call the destructor -
and that only makes sense for structs and classes.
Allowing it for other types (especially pointers) just sneaks in
non-obvious bugs, especially when it's considered a replacement for
delete (which calls the destructor for both Object a *struct).

BTW: clear() has often been mentioned in this NG but isn't documented in
the Phobos documentation (which is no surprise because clear() doesn't
have doc-comments).

So I guess I'll report this as a bug.

Cheers,
- Daniel


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