🚀 go-pugleaf

clamage@Eng.Sun.COM (Steve Clamage) writes:

>In article 460@news.bridge.net, David Byrden <100101.2547@compuserve.com>
>writes:
>>Why, in the Septenber draft standard, is an allocator-specific verrion of 
>>'new' provided;
>>
>>20.1.4.4
>>
>>           new(x) T    
>>
>>    returns an X::types<T>::pointer, where x is of an 
>>    allocator type X

>You are actually referring to the "placement new" syntax not having a
>corresponding "placement delete" syntax.

>No good syntax was found for such. The obvious syntax
>	delete (x) p;
>leads to too many ambiguities. More importantly, it means that the
>point of deletion would somehow have to know what placement version
>of "new" was used and what the parameters were. Needing that knowledge
>would make "placement delete" unsafe and hard to use. (Consider the
>general case where the "new" and "delete" expressions are in different
>compilation units, and the parameters to "new" affect how "delete"
>should be called.)

>You can define a version of "operator delete()" corresponding to a
>placement new. If the constructor for the object exits via an exception,
>the compiler will call the corresponding placement delete automatically
>as a special case.
>	T* p = new (x) T;
>The reasons for this special case are
>1. You do not know what needs to be deleted in general, but the compiler
>does.
>2. The compiler has all the information it needs to invoke the special
>operator delete() correctly. It is invoked at the point of the
>new-expression.

>>but there is no matching version of 'delete'? Are we supposed to use both 
>>destroy() and deallocate() to get rid of that object?

>Yes. In the above example:
>	p->~T();
>	operator delete(p, x);

How can you guarantee that the first argument to operator delete
will be correct? It might be slightly safer to write:

	void* q = p;  // evaluate before destruction
	p->~T();
	operator delete(q,x);

and I am not sure if this is guaranteed to be safe.

One form for placement operator delete, that might avoid ambiguity is:

delete (placement parameters) (pointer)

The extra parentheses around the pointer would make this not a
syntactically correct non-placement delete.
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