Re: bstr and delete[]

From:
yufufi <yufufi_at_comcast.net>
Newsgroups:
comp.lang.c++
Date:
Tue, 20 Feb 2007 14:14:50 -0800
Message-ID:
<O8qdnfufh4xC70bYnZ2dnUVZ_revnZ2d@comcast.com>
mlimber wrote:

On Feb 20, 2:38 pm, yufufi <yufufi_at_comcast.net> wrote:

How does delete[] know how much memory to deallocate from the given
pointer? AFAIK this informations is put there by new[]. new[] puts the
size of the allocated memory before the just before the beginning of the
array. But I couldn't find this information by looking at the memory.
(Via VS2005 - C++)


http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.14

My second questions is, if there is a mechanism to know how much memory
is allocated for the array, why don't we use it for things like bstr?
Only thing I can think of is you may go and allocate a big chunk of
memory with new[] and use it for more then one bstr's and such. (However
  i'm not even sure how you can do it with bstr since you have to use
::SysAllocString)


This part is off-topic here. Try a Microsoft group (cf. the list at
http://www.parashift.com/c++-faq-lite/how-to-post.html#faq-5.9)

Cheers! --M


The page you point to answers the question of "How many objects to
destruct?" which is a little bit different then what I'm asking.

In the sample code for one of the delete[] implementations:

// Original code: Fred* p = new Fred[n];
  char* tmp = (char*) operator new[] (WORDSIZE + n * sizeof(Fred));
  Fred* p = (Fred*) (tmp + WORDSIZE);
  *(size_t*)tmp = n;
  size_t i;
  try {
    for (i = 0; i < n; ++i)
      new(p + i) Fred(); // Placement new
  }
  catch (...) {
    while (i-- != 0)
      (p + i)->~Fred(); // Explicit call to the destructor
    operator delete[] ((char*)p - WORDSIZE);
    throw;
  }

'n' used to decide number of destructor calls needed. But in the line
one before the last:

operator delete[] ((char*)p - WORDSIZE);

Still compiler is presented with a pointer to an array(which in it's
first WORDSIZE has n)

    1) Isn't this an infinite loop(recursive)?
    2) If the second one is somehow a different delete[], will it act like:
    operator delete[] (*p)
    {
        arraySize = (((WORDSIZE *)p)[0])*sizeOf(Fred)
        InternalDelete(p,arraySize);

    }

Thank you,

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