Re: why isn't there a placement delete syntax

From:

Ulrich Eckhardt <eckhardt@satorlaser.com>

Newsgroups:

comp.lang.c++.moderated

Date:

27 Sep 2006 09:01:25 -0400

Message-ID:

<b2lpu3-bfi.ln1@satorlaser.homedns.org>

andrew_nuss@yahoo.com wrote:

I have created an interface with placement new that uses a Heap
reference that supplies Alloc/Free functionality. The compiler forces
the definition of a placement delete. But there is no syntax that I
can find to invoke the placement delete!

struct Object {
    static void* operator new (Heap&, size_t);
    static void operator delete (Heap&, void*);
};

class MyObj : public Object {
    ...
};

main {
    Heap heap;
    MyObj* p = new (heap) MyObj(); // works!
    delete (heap) p; // illlegal!
}

Can anyone explain how to delete these objects. I.e. why doesn't the
obvious analagous invocation of delete work using placement syntax.

I'm not sure exactly what you are puzzled about, but there is in fact lots
of symmetry here. The point is that the normal new in fact allocates
storage and creates an object (invokes the ctor) in that memory. The normal
delete first destroys it and then releases the memory.
Now, placement new only removes the task of allocating storage, the storage
is already provided. Similarly, you only invoke the destructor lateron
instead of calling delete because the memory management is left out.

The syntax for those steps is

  char buffer[sizeof (X)];

  X* x = new (buffer) X;
  x->~X();

I guess that what puzzles you is the fact that those two operations use a
widely different syntax to achieve their goals. I agree that that's not too
beautiful.

Uli

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