TITLE: using placement delete

(Newsgroups: comp.std.c++, 27 Aug 97)

ISJ: isj@image.dk

> I do not completely understand how placement works on operator delete.
> 
> The December '96 Working Paper contains a lot of references to
> "placement operator delete" in 3.7.3.2, 5.3.4(.18), 15.2(.2) and
> 18.4.1.3(.2) and explains much about them (parameters, exceptions,
> prototype etc..). But I cannot find a single example of calling an
> placement operator delete.
> 
> The grammar of delete-expression in 28.4 does not allow a
> "delete-placement" (neither does the grammar in "C++ prog.lang." 3rd
> ed.).


CLAMAGE: Steve Clamage <stephen.clamage@eng.sun.com>

Let's be sure we are not confusing a delete-expression, as in
	delete p;
with a function whose name is "operator delete".

There is no delete-expression which will invoke a placement
form of operator delete. You can still call the function
explicitly. Example:

	// a pair of placement operators
	void operator delete(void*, int);
	void* operator new(size_t, int);

	T* p = new (12) T; // invoke placement new
	...
	// destroy object and delete space manually
	p->~T(); 
	operator delete(p, 12); 

If you just wrote
	delete p;
it would use the normal operator delete, which presumably
is not correct, since you took the trouble to declare a
form to match the placment new.

The placement operator delete is called automatically
ONLY in the case when a constructor exits via an exception
and was invoked by using a placement expression.

For example, suppose the constructor for T above exited via
an exception. Since there is no way for the programmer to
know how much got constructed, the compiler is required
to generate code that automatically destroys the already-
constructed parts and deletes the allocated space. If there
is a placement form of operator delete that matches the
placement operator new, it is called. Otherwise, the normal
operator delete is called.

Only local information is needed for the compiler to be able
to take care of the situtation. The compiler generates code to
trap an exception and pass the same extra values to placement
operator delete that it passed to placement operator new.

ISJ:

> ...  It is also more-or-less
> contradicted by "C++ prog.lang." page 256, section 10.4.11 ...

CLAMAGE:

The 2nd edition of C++PL was written before placement
delete was added to the language definition. The example
you cite is still valid. Since placement delete was not
available, a more complicated solution was used than
is now needed.