Re: Placement new and explicit destruction

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 19 Jul 2007 07:52:54 -0000

Message-ID:

<1184831574.679076.55840@e16g2000pri.googlegroups.com>

On Jul 17, 5:43 pm, REH <spamj...@stny.rr.com> wrote:

On Jul 17, 5:15 am, James Kanze <james.ka...@gmail.com> wrote:

On Jul 16, 11:35 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

REH wrote:

This curiosity popped into my head on the way to work today. I
wouldn't actually do this, but just wondering. Is the following
defined behavior?
#include <new>
class T { };
int main()
{
   T* p = new T();
   p->~T();
   new(p) T();
   delete p;
   return 0;
}

Yes. It's the pattern some folks are using to "forward constructor
calls". The functionality is so often asked for that the Committee
is changing the language to allow forwarding constructors, BTW.

It's definitly legal, but I don't quite see what it has to do
with forwarding constructors. If I've understood the proposals
correctly, a forwarding constructor is where one constructor
first calls another constructor to do the job, then does some
additional processing. Here, the poster first allocates and
constructs the object, then destructs it without deallocating
(leaving raw memory), then reconstructs it in the raw memory.

I don't want to speak for Victor, but what I think he was saying was
that placement new can be used to simulate one constructor calling
another.

Except that it can't, in general.

Thus:

class foo {
public:
  foo(int i);
  foo(double d);
};

foo::foo(double d)
{
  new(this) foo(123);

Which is undefined behavior if the class has a non-trivial
destructor.

}

The reason why the standards committee is adding forwarding
constructors is precisely because there is no way of simulating
it in the current language.

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James Kanze (GABI Software) email:james.kanze@gmail.com
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