From: "Duane Murphy" <duanemurphy@mac.com> Reply-To: Boost-Users@yahoogroups.com To: "Boost Users" <Boost-Users@yahoogroups.com> 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 :) _________________________________________________________________ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail