Re: [Boost-users] [range] const-only UDT works as std algos, but not with boost range.

8 Sep 2016


      Thanks, Why does boost range require a public non-const iterator that will never 
be used, yet the std:: framework does not? Is this intentional?


On 09/08/2016 08:08 AM, Neil Groves wrote:
...
On 7 September 2016 at 22:18, John
 wrote:
...
A class that exposes a const_iterator, but does not expose a mutable
iterator, works with std:: algorithms and range-for() loop, but does not
work with boost range.
This is true because it does not meet the requirements of the
Boost.Range Concepts (see
http://www.boost.org/doc/libs/1_61_0/libs/range/doc/html/range/concepts/sing...).
To range requires a boost::range_iterator<T> and a
boost::range_iterator<const T>. There are several solutions. The
typical one is to simply add using iterator = const_iterator into the
type to model the range concept. However if one requires a
non-intrusive solution one can provide a specialization of
boost::range_iterator. This design decision for the concepts means
that there need not be const and mutable concepts for the ranges. It
also makes the selection of overrides work with typical models of the
Range Concepts such as Containers.
...
// This works with std algorithms and range-for, but not with boost range.
class C {
public:
    typedef blah const_iterator;
    const_iterator begin() const;
    const_iterator end() const;
};
const C c;
for (auto x : c) { }  // OK
std::for_each(std::begin(c), std::end(c), fn);  // OK
boost::for_each (c, fn); // ERROR: fails SinglePassRange concept (GCC-6.2)
The workaround is to add a non-const iterator type that is the same as the
const iterator, and to add two additional non-const begin/end methods.
This is more than necessary to fix the problem. You only need to add
using iterator = const_iterator; into class D. The additional begin()
and end() functions that return iterator are not required.
...
class D {
public:
    typedef blah const_iterator;
    const_iterator begin() const;
    const_iterator end() const;
// kludge to make it work with boost.
    typedef const_iterator iterator;
    iterator begin();
    iterator end();
};
const D d;
boost::for_each (d, fn); // OK
Is there a simpler solution?
Yes, as described above all you need is one line so it models a Range
Concept. In other scenarios you might want to look at boost::sub_range
which will preserve const-ness from the wrapped type while providing
both const_iterator and iterator types
(http://www.boost.org/doc/libs/1_36_0/libs/range/doc/utility_class.html).
...
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I hope this helps.
Regards,
Neil Groves
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