Re: I cannot see the need for auto_ptr?!

From: Karl Heinz Buchegger (kbuchegg_at_gascad.at)
Date: 10/20/03 

Date: Mon, 20 Oct 2003 10:09:42 +0200

grejdanospam@pacbell.net wrote:
>
> >
> > void bar()
> > {
> > std::auto_ptr<LargeObject> p = foo();
> >
> > p->doTheFandango();
> > }
> >
> > where the call to 'foo' transfers ownership up to the local
> > std::auto_ptr in 'bar', which guarantees deallocation even if, as Murphy
> > guarantees will happen, 'doTheFandango' throws an exception.
> >
> >
>
> Yes, but returning a pointer from foo is a bad progamming practice.

Says who?
Sometimes you don't have a choice.

> Why don't you just say that auto_ptr is deprecated ?

You are concentrating too much on returning pointers.

void bar( int Type )
{
  Obj* pObj;

  pObj = Factory.CreateObject( Type );

  // do something

  // object no longer needed, delete it.

  delete pObj;
}

Now lets say, that the part // do something is rather complicated and there
is a chance that things might go wrong. You insert some tests and do a return.
But when doing so you must not forget do delete the object given to you
from the factory.

 
void bar( int Type )
{
  Obj* pObj;

  pObj = Factory.CreateObject( Type );

  // do something
  ...
  if( SomeTest ) {
    delete pObj;
    return;
  }

  if( SomeOtherTest ) {
    ...
    while( ... ) {
      if( AThirdTest )
        return;
    }
  }
  // object no longer needed, delete it.

  delete pObj;
}

So you see the big? Deep inside the while, there is an if which returns. It happened
to me, that I forgot to delete pObj, because I am human :-)

An auto_ptr saves me from all of this. I don't need to remember to delete pObj, it
is done automatically for me. For the very same reason that you use vector instead
of dynamically allocated arrays, you use an auto_ptr when you must use a pointer, to
free yourself from human errors and to easen maintenance (who says that this test
was there in the first place. It could have been added weeks later). 

-- 
Karl Heinz Buchegger
kbuchegg@gascad.at

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