Re: XMLURL memory allocation problem

[Sam Lindley]

I was wrong... This isn't a problem with VC++. It is just following the 
usual overloading rules, and placement new is hidden. The difference between 
VC++ and gcc is that VC++ uses placement new in its implementation of STL 
containers such as vector. This seems like a perfectly reasonable 
implementation decision.

The fix appears to work: add placement new (and delete) to the XMemory 
class. Is there any reason why this hasn't been done already?

Sam
----- Original Message ----- 
From: "Sam Lindley" <[EMAIL PROTECTED]>
To: <xerces-c-dev@xml.apache.org>
Sent: Friday, December 24, 2004 1:46 AM
Subject: Re: XMLURL memory allocation problem


I think I've found the root of the problem, and it seems to be something 
that's been noticed before, e.g:

 http://marc.theaimsgroup.com/?l=xerces-c-dev&m=107349519306035&w=2
AFAICS, VC++ (2003) doesn't handle overloading of operator new properly. 
The XMemory class overloads placement new for allocating XMemory objects 
using a memory manager:

 void* operator new(size_t size, MemoryManager* memMgr);
but the STL uses placement new, where the second argument is a void 
pointer:

 void* operator new(size_t size, void* p);
I haven't checked, but I believe the correct behaviour (according to the 
C++ standard) should be that the original placement new (with void 
pointer) remains visible. This appears to be what happens in the gcc 
(3.4.3), but not in VC++. It seems that overloading new for a particular 
class obscures all of the original global overloadings of new with respect 
to that class (even when not all of the versions of new have been 
overloaded in the class). The easy workaround is to copy any of the 
original global definitions which haven't already been overloaded. In the 
case of the XMemory class, the void* variant of placement new can be 
overloaded using the original definition:

 void* operator new(size_t size, void* p) {return p;}
Now, I seem to have solved the problem... but was it actually safe to use 
XMemory objects in STL containers in the first place?

Sam
----- Original Message ----- 
From: "Sam Lindley" <[EMAIL PROTECTED]>
To: <xerces-c-dev@xml.apache.org>
Sent: Wednesday, December 22, 2004 1:33 PM
Subject: XMLURL memory allocation problem


The following code:
#include <vector>
#include <xercesc/util/XMLURL.hpp>
using namespace std;
XERCES_CPP_NAMESPACE_USE
int main()
{
 vector<XMLURL> urls(1, XMLURL());
 return 0;
}
fails to compile under Visual C++ 2003 (VC_7_1) using xerces-c 2.6.0. It 
gives the error:

 C2665: 'xercesc_2_6::XMemory::operator new' : none of the 3 overloads 
can convert parameter 2 from type 'void *'

Apparently the same code compiles fine under gcc 3.4.3 and xerces-c 
2.6.0. The same problem occurs with XMLUri (and presumably any class 
derived from XMemory). If vector is changed to list, then it compiles.

Any suggestions?
Sam

---------------------------------------------------------------------
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]