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

--- 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. 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 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