Re: [Boost-users] shared_ptr and container iterators

Subject: Re: [Boost-users] shared_ptr and container iterators
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-04-17 12:23:59

On Tue, Apr 17, 2012 at 8:56 AM, Johannes Totz <johannes_at_[hidden]> wrote:

> On 16/04/2012 20:06, Igor R wrote:
> >> just started playing with boost::shared_ptr.
> >> How do I get a const iterator pointing to a shared_ptr pointing to a
> >> const object? See code below, I'd like to prevent a MyItem being changed
> >> through const iterators.
> >
> > IIUC, the following answers your question:
> > http://www.boost.org/doc/libs/1_49_0/libs/smart_ptr/shared_ptr.htm#FAQ
> > <<Q. Why is operator->() const, but its return value is a non-const
> > pointer to the element type?
> > A. Shallow copy pointers, including raw pointers, typically don't
> > propagate constness. It makes little sense for them to do so, as you
> > can always obtain a non-const pointer from a const one and then
> > proceed to modify the object through it.shared_ptr is "as close to raw
> > pointers as possible but no closer".>>
>
> Got a recommendation as to how to get this const-correct? I'm kinda
> clueless right now... Or does the above faq mean it just cant be done?
>
>
> struct MyItem
> {
>        int     value;
> };
>
> struct MyContainer
> {
> protected:
>        typedef std::set<boost::shared_ptr<MyItem> >    MySet;
>        MySet   set;
>
> public:
>        MySet::iterator stuff_begin();
>
>        // should not change ptr nor MyItem nor set
>        XXX::const_iterator stuff_begin() const;
> };
>

Maybe you can use a transform_iterator to map the shared_ptr<MyItem> to a
shared_ptr<MyItem const> (if such a conversion exists, I'm not sure)?

- Jeff