Re: why isn't there a placement delete syntax

"kanze" <>
27 Sep 2006 09:14:03 -0400
Alberto Ganesh Barbati wrote: ha scritto:

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 believe a placement delete syntax has not been provided
because it could be too error-prone. It would be too easy to
make the mistake of invoking delete with a parameter value
different from that used for new.

What do I do?

Placement new is expected to store the additional info
somewhere, in particular in a place where operator delete can
obtain it using the only information it has, i.e.: the

Note that this means that you cannot define a placement new
requiring a delete without also replacing global new and delete.
You have to replace global delete, since this is the delete
which will be called even for the objects allocated by placement
new. And you have to replace global new, because global new has
to work with global delete (and there's no way for your new
global delete to access the global delete it is replacing).

The most obvious way is to store the info in block itself, as
in (alignment issues omitted for brevity):

void* operator new (Heap& h, size_t n)
   // alloc some more space to hold the extra info
   void* ptr = h.alloc(n + sizeof(Heap*));
   Heap* hptr = static_cast<Heap*>(ptr);
   *hptr = &h; // store info in the block
   return hptr + 1;

// placement delete used only when the constructor throws
void operator delete (Heap& h, void* ptr)
   Heap* hptr = static_cast<Heap*>(ptr); - 1);

// regular delete, not placement!
void operator delete (void* ptr)
   Heap* hptr = static_cast<Heap*>(ptr);
   hptr[-1]->free(hptr - 1); // retrieve info from the block

And what happens when this delete is called for an object
allocated with non placement operator new?

A more typical solution would involve replacing the global
operator new as well, having it also allocate the extra memory,
but set the heap pointer to NULL. Then the global operator
delete becomes:

    operator delete( void* p )
        Heap* hptr = static_cast< Heap* >( p ) - 1 ;
        if ( hptr == NULL ) {
            free( hptr ) ;
        } else {
            (*hptr)->free( hptr ) ;

(This assumes you've used malloc to allocate memory in the
replacement global operator new, of course.)

James Kanze GABI Software
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