Re: [Boost-users] Modifying all matching elements in a Boost.MultiIndex Container

Subject: Re: [Boost-users] Modifying all matching elements in a Boost.MultiIndex Container
From: Amit Prakash Ambasta (amit.prakash.ambasta_at_[hidden])
Date: 2015-12-31 15:55:58

Hey Joaquin

First of all, thank you so much for your repeated help.
In line with the example provided, I've written a generic find_and_modify
function as:

        template <typename T, typename F> Segment* find_and_modify(T&
iterable, F filters, State s, double a_arr=-1, Comment
rmk=Comment::INFO_SEGMENT_PREDICTED) {
            auto iter = iterable.find(filters);

            if (iter == iterable.end())
                return nullptr;

            std::pair<typename T::iterator, typename T::iterator> iter_main
= iterable.equal_range(filters);

            while (iter_main.first != iter_main.second) {
                auto it = boost::multi_index::project<aux>(segment,
iter_main.first);
                segment.get<aux>().modify(
                    it,
                    [&s, &a_arr, &rmk](Segment& seg) {
                        seg.state = s;
                        if (a_arr != -1)
                            seg.a_arr = a_arr;

                        if (rmk != +Comment::INFO_SEGMENT_PREDICTED) {
                            seg.comment = rmk;
                        }
                    }
                );
                iter_main.first++;
            }

            return (Segment *)(&(*iter));
        }
where T is an index on container instance segment as segment.get<sometag>();

This seems to work well for the most part but it still segfaults. Any ideas
what I'm doing wrong. As specified in the SO answer, I've created an aux
index and projected the current iterator to it so as to not modify the
current iterator, and yet on debugging the function, I've found that
iter_main.first evaluates to garbage at some point as:

(gdb) p (*iter_main.first)
$7 = (const boost::multi_index::detail::index_node_base<Segment,
std::allocator<Segment> >::value_type &) @0x6b3c10: {
  index = "
\252\355\000\000\000\000\000\020\376u\000\000\000\000\000\060\355u\000\000\000\000\000\020\376u\000\000\000\000\000\320\361j\000\000\000\000\000\360:r\000\000\000\000\000\270۾\366\377\177\000\000\270۾\366\377\177\000\000\310۾\366\377\177\000\000\310۾\366\377\177\000\000\330۾\366\377\177\000\000\330۾\366\377\177\000\000\260\363u\000\000\000\000\000\260\363u\000\000\000\000\000`;k\000\000\000\000\000`;k\000\000\000\000\000\200\227j\000\000\000\000\000\200\227j\000\000\000\000\000\030ܾ\366\377\177\000\000\030ܾ\366\377\177\000\000(ܾ\366\377\177\000\000(ܾ\366\377\177\000\000\070ܾ\366\377\177\000\000\070ܾ\366\377\177\000\000Hܾ\366\377\177\000\000"...,
code = "OP", cname = "34283", p_arr = 1450614300, p_dep = 1450667229, a_arr
= 3, a_dep = inf, t_inb_proc = 3600, t_agg_proc = 0, t_out_proc = 18000,
cost = 0,
  state = {static _size_constant = 5, _value = 3 '\003'}, comment = {static
_size_constant = 9, _value = 2 '\002'}, parent = 0x75ed70}

On Fri, Jan 1, 2016 at 1:03 AM Joaquín M Lopez Munoz <joaquin_at_[hidden]> wrote:

> Amit Prakash Ambasta <amit.prakash.ambasta <at> gmail.com> writes:
>
> >
> > std::pair<index::iterator, index::iterator> iter =
> index.equal_range(value);
> >
> > while(iter.first != iter.second) {
> >
> >
> >   index.modify(
> >     iter.first,
> >     [](T& element) { ... }
> >   );
> >
> >   iter.first++;
> > }
> >
> > leads to segfaults and unexpected behavior.
> >
> > What would be right way to approach this?
>
> This is a classic, please follow these links for further info:
>
> http://stackoverflow.com/questions/29013600/
>
> modifying-key-ranges-in-a-boost-multi-index-map-having-hashed-non-unique-keys
> http://lists.boost.org/boost-users/2006/03/18048.php
>
> Joaquín M López Munoz
> Telefónica
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users