Re: [boost] Constant iterator of Single Pass Range

Subject: Re: [boost] Constant iterator of Single Pass Range
From: Neil Groves (neil_at_[hidden])
Date: 2014-05-05 07:40:05

On Mon, May 5, 2014 at 11:03 AM, Jan Hudec <bulb_at_[hidden]> wrote:

> Hello All,
>
> The documentation of [Single Pass Range][1] in Boost.Range says that there
> should be both `iterator` and `const_iterator` associated types and both
> const and non-const overloads for `begin` and `end`. However if it is
> really a single-pass range, I don't see a way to implement a
> `const_iterator` that would work on `const` reference to the range, because
> the `operator++` of the iterator involves calling a non-const method of the
> range.
>

I think that your statements indicate that you have a misunderstanding
about implementing iterators. The word "const" does not refer to the
iterator type itself but to the reference type. It is not possible without
resort to very perverse use of the "mutable" keyword to implement a
const_iterator to refer to any type. Of course, const_iterator is part of
every standard container so this is not an idiom that is specific to
Boost.Range.

> For example the `std::istream_iterator::operator++` calls
> `std::istream::operator>>`, which is non-const and so there is no
> `const_istream_iterator`. The `istream_range` cheats around this a little,
> because the range
> is a wrapper around the stream, not the stream itself. But when I have
> similarly behaving class that I can make itself be a range, do I really
> have to work around this using a wrapper, or can the requirement be relaxed
> in practice?
>

I believe that you don't have a problem! A const_iterator can be
implemented with non-const member functions. There are examples within the
Boost.Range unit test code that show the use of extending Boost.Range for
user defined types. These show the various methods for implementing this.
There is also documentation that has an example extending via the const and
mutable meta-functions here:
http://www.boost.org/doc/libs/1_55_0/libs/range/doc/html/range/reference/extending/method_2.html

In the above example the type returned from the meta-function is the same
for both the const and mutable versions. If the iterators are acting upon a
container then you often don't need to provide either of the range
meta-functions for obtaining the iterator since the member typedefs will be
sufficient. I typically implement my own container types to simply have the
member typedefs and avoid the need to specialise the meta-functions. One
would typically extend the meta-functions when the custom range or
container does not model standard container Concepts. It might therefore be
worth reviewing the rest of the Boost.Range reference section on extending
the library.

>
> --
> Jan Hudec <bulb_at_[hidden]>
>

If I have misunderstood your problem, or you require any additional help
please let me know.

Regards,
Neil Groves