[Digikam-devel] Re: delete does not reset the pointer in GCC (4.6.0)

[Francesco R.]

On Wednesday 15 June 2011 14:17:37 Gilles Caulier wrote:
> 2011/6/15 Andi Clemens <andi.clemens at gmx.net>
> 
> > Right, we are still using too many raw pointers.
> > Qt, C++ stdlib and boost have many things to handle pointers
> > automatically.
> 
> Yes. this one is exactly that we need :
> 
> http://doc.qt.nokia.com/4.7/qpointer.html
> 
> > See Item #13 in "Effective C++": Use objects to manage resources
> > A great book, everyone should read it...
> 
> Do you have an url ?

This may be of interest:
http://labs.qt.nokia.com/2009/08/25/count-with-me-how-many-smart-pointer-
classes-does-qt-have/

for some more fun I've converted the previous example to use QPointer and 
QObject, it's at the end of the mail.

> 
> > Anyway sure auto_ptr and friends will "slow" down execution, but with
> > all
> > the checks we do anyway, I guess it will not be noticeable.
> > It is still much faster than garbage collection...
> 
> Perhaps we will need to patch code about to use QPointer everywhere when
> it's necessary...
> 
> Gilles
> 
> > Andi
> > 
> > 
> > On Wed, 15 Jun 2011 11:55:50 +0200, Marcel Wiesweg
> > <marcel.wiesweg at gmx.de>
> > 
> > wrote:
> > >> a few months ago, maybe 2 years.
> > >> I know that I always added
> > >> xyz = 0;
> > >> to my code before, but then it was removed again because it is not
> > >> neccessary with modern compilers (I don't know who said that).
> > > 
> > > When googling and reading through the usual sources (stackoverflow
> > > etc.) I see noone saying that any compiler sets a pointer to 0, I
> > > would even guess delete should _not_ change the value of the passed
> > > pointer unless the C++ standard says so.
> > > There are the usual stories not to use raw pointers and use concepts
> > > to
> > > make sure you are always sure never to use deleted pointers again,
> > > but
> > > in the end, if you are playing with raw pointers (which we are doing
> > > still in too many areas IMO, but getting less) and need to delete
> > > and
> > > reuse the variable, be sure to set it to 0.
> > > 
> > > Marcel

#include <iostream>
#include <QtCore/QObject>
#include <QtCore/QPointer>

/*
g++-4.5.2 -I /usr/include/qt4 -L/usr/lib64/qt4 -lQtCore -O0 -ggdb3 -fdump-
tree-gimple b.cpp && mv b.cpp.004t.gimple b.cpp.004t.gcc45.gimple
g++-4.6.0 -I /usr/include/qt4 -L/usr/lib64/qt4 -lQtCore -O0 -ggdb3 -fdump-
tree-gimple b.cpp && mv b.cpp.004t.gimple b.cpp.004t.gcc46.gimple

# to compare use:
sed -i -e 's:[0-9][0-9][0-9][0-9][0-9]:99999:g' b.*.gimple

*/

using namespace std;

// QPointer<QLabel> label = new QLabel;
//      label->setText("&Status:");
//      ...
//      if (label)
//          label->show();

main()
{
        QPointer<QObject> p = new QObject;
        p->dumpObjectInfo();

startLookingAtMe:
        delete p;
        // p = 0; // omitting DOES NOT crash
        delete p;
        if (p) delete p;
endLookingAtMe:
	cout << (0/0); // my easy brakpoint
}