Re: [Boost-users] std::vector< boost::shared_ptr<int> >::pop_back()

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2006-07-10 09:43:45

On Mon, July 10, 2006 07:58, bringiton bringiton wrote:
> This question is based on curiosity. How does std::vector::pop_back() call
> the destructor of the item getting removed?
>
> i understood std::vector to be a contigious array of memory, therefore an
> item's memory does not go out of scope when being popped. ie the item goes
> out of scope when the entire array goes out of scope.
>
> example code: the reference count is decremented when the item is popped
> from the vector.
>
> boost::shared_ptr<int> item1(new int(1));
>
> std::vector< boost::shared_ptr<int> > vect1;
> vect1.push_back(item1);
>
> std::cout << item1.use_count() << std::endl;
> vect1.pop_back();
> std::cout << item1.use_count() << std::endl;
>

vect1.pop_back() removes an element at the end of the vector.
So the element is not there anymore. This is done using the std::allocator class. Even if item's
memory remains (I assume a vector implementation can decide whether to keep the last chank for
later insert or not), allocator calls destroy and the element is destroyed (its destructor is
called, if it is a compound type (the case with shared_ptr)). If destructor of shared_ptr is
called its use_count is decreased.

With Kind Regards,

Ovanes Markarian