c++/9982: placement new returns wrong value

[Thomas Kunert]

The following reply was made to PR c++/9982; it has been noted by GNATS.

From: Thomas Kunert <kunert@physik.tu-dresden.de>
To: bangerth@dealii.org, gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org,
        kunert@physik.tu-dresden.de, nobody@gcc.gnu.org, gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: c++/9982: placement new returns wrong value
Date: Fri, 07 Mar 2003 14:21:54 +0100

 bangerth@dealii.org wrote:
 > Synopsis: placement new returns wrong value
 > 
 > State-Changed-From-To: open->closed
 > State-Changed-By: bangerth
 > State-Changed-When: Thu Mar  6 19:41:07 2003
 > State-Changed-Why:
 >     This is not a bug. For array expressions, the compiler puts
 >     the number of array elements into the first slot (pointed to
 >     by the placement argument), and only then the elements of the
 >     array. This is so because delete[] needs this information.
 >     
 
 There is no delete[] corresponding to the placement new[], hence this is 
 useless. In many cases the compiler actually seems to figure that out:
 changing the line
 
      A  * b = new(a) A[3];
 
 to
 
      A  * b = new((void*)a) A[3];
 
 makes a and b pointing to the same position. Why is the difference here?
 
 But you are right, the standard seems to allow any behavior. However, 
 this makes operator new(a)[] pretty much useless, since the final 
 position of the new array is undefined and one does not even know the 
 necessary size of the provided memory region.
 
 Thomas
 
 
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9982