Re: [Boost-Users] Newbie shared_ptr in a container question.

From: Joshua Little (animedemon_at_[hidden])
Date: 2003-07-27 20:28:41

>From: "Duane Murphy" <duanemurphy_at_[hidden]>
>Reply-To: Boost-Users_at_[hidden]
>To: "Boost Users" <Boost-Users_at_[hidden]>
>Subject: Re: [Boost-Users] Newbie shared_ptr in a container question.
>Date: Sun, 27 Jul 2003 09:17:48 -0700
>
>--- At Sat, 26 Jul 2003 19:57:17 +0000, Joshua wrote:
>
> >I've been reading through posts on here and in the docs but I think I may
> >have missed something. Heres what I am trying to do:
> >lets say I have a class Foo* and I'm trying to use it in a
> >std::vector<shared_ptr<Foo> >. I can add them all day long and it works
> >fine. The problem is that when I try to remove the shared_ptr from the
> >vector. I expected it to delete the memory for the Foo* then delete
> >itself (to completely free the memory that the shared_ptr used and the
> >memory that the Foo was using. I think this is not happening though since
> >the destructor is not being called when the elements are removed from the
> >vector. Heres how I am currently inserting the shared_ptr's into the
> >vector (yes I know this is the wrong way now that I've read a few of the
> >posts here):
> >
> >typedef std::vector<boost::shared_ptr<Foo> > FooVector;
> >
> >FooVector Foobar;
> >Foo* blah = new Foo();
> >boost::shared_ptr<Foo> * NewPointer = new boost::shared_ptr<Foo>(blah);
> >Foobar.insert(*NewPointer); //Yes I know this looks horrible. And its
> >probably wrong.
> >
> >
> >When I call Foobar.erase() I thought that the shared_ptr would be removed
> >and the memory freed (and Foo's destructor called). I tried it like this 
>:
> >FooVector::iterator CurrentFoo = Foobar.begin();
> >Foobar.erase(CurrentFoo);
> >
> >That did remove the shared_ptr but the Foo* did not seem to get
> >deallocated, its destructor was never called. Some of the posts seem to
> >indicate that this is a very bad way to let shared_ptr manager the
> >memory, is there a better way to do what I want to do using shared_ptr or
> >is shared_ptr the wrong tool to use?
>
>There have been many good suggestions as to what the correct sequence for
>inserting the shared_ptr into the vector. I'm not sure the real problem
>has been adequately explained. Why isnt the foo* being deleted.
>
>Shared_ptr<> counts the number of instances of the pointer that are
>created. When you did:
>
>boost::shared_ptr<Foo> * NewPointer = new boost::shared_ptr<Foo>(blah);
>
>You created the first instance; count == 1.
>
>When you did:
>
>Foobar.insert(*NewPointer); //Yes I know this looks horrible. And its
>
>A copy of the shared_ptr<> was made. The count is now 2.
>
>When you did:
>
>Foobar.erase(CurrentFoo);
>
>One copy (of two) was "deleted". In this sense, the shared_ptr<> was
>deleted. The destructor for the shared counter mearly decrements the
>counter. Count == 1.
>
>The problem is that the original count was leaked by using "new
>shared_ptr<>". This is why all the suggestions show that you should be
>using the stack based method for shared_ptr<>.
>
>Shared_ptr is almost never allocated from the heap. There really is not
>much point. It's use is as a value representation of pointer.
>
>I hope this helps you understand where your code went astray.
>
>Good luck,
>  ...Duane
>
>
>
>
>  ...Duane
>
Reading many of these replys I now see why what I was doing wasn't working, 
so I'm going to change my code around and hopefully get this cleared up. 
Thanks to everyone for your help :)

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