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Subject: [Boost-users] [Range, Container] range of to-be-moved values ?
From: John M. Dlugosz (ngnr63q02_at_[hidden])
Date: 2014-02-05 04:49:46

Given a boost::container::vector<C> vec;
I can write vec.push_back(move(value)) for a move-only (non-copyable) type C, instead of
vec.push_back(value) as usual.
But what would I write instead of vec.insert(pos,b,e) where (b,e] is a range of values?
I'm particularly fond of Range concept, rather than loose iterators, so my function would
be something that takes something that works as a Range; e.g.

        template <typename R>
        void prepend (const R& src_range)
                auto b= boost::begin(src_range);
                auto e= boost::end(src_range);
                vec.insert(vec.begin(), b, e);

An rvalue reference to a range is not the same thing as a range of values that I may move
from. I could have a differently named function and pass it a range with the implicit
assumption that the function will modify the values, but is there a proper way to indicate
that as part of the type? src_range can be, say, a pair of (non-const) iterator s as
opposed to a pair of const_iterator s, or otherwise range_iterator<R>::type is
dereferencable to a const value or not. But that itself is hard to see in general let
alone overload against!

If my function were called prepend_and_destroy_original(src) it might clue in the caller,
but in general it is surprising that a parameter of (const R&) will modify the values.
src itself doesn't change, and now refers to the same range of values which are now
swapped out with defaults. With built-in pointers and even shared_ptr I can distinguish
between an in-out parameter and a parameter that dereferences to a modifiable or const
value. But what about with Ranges?

Even with lower-case ranges provided by two iterators, How can you call something like
        boost::container::vector v2 (InIt_start, InIt_end);
to convert a std::vector or some other container of move-only items? Only if the types
match exactly can the vector(vector&&) constructor be used.


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