Re: [Boost-users] Why must a Single Pass Range be default constructible?

Subject: Re: [Boost-users] Why must a Single Pass Range be default constructible?
From: Neil Groves (neil_at_[hidden])
Date: 2009-04-22 06:42:44

Hello, thank you for your comments.

On Tue, Apr 21, 2009 at 4:31 PM, athor <thorena_at_[hidden]> wrote:

>
> A Range does not require copy semantics according to the boost docs.
> http://www.boost.org/doc/libs/1_38_0/libs/range/doc/range.html
>
> The SinglePassRangeConcept (and all range concepts using it) requires a
> range to be default constructible. (There is a line "T a;" in the concept
>

> The default constructible requirement means an abstract base class can
> never
> implement any Range Concept. If a range does not require any copy
> semantics,
> I don't see why it must be default constructible? Are there any good
> reasons
> I don't know about?
>

I (Neil Groves) am the current developer of the next version Boost.Range
known as Boost.RangeEx I fully intend to release a new version without the
default constructible requirement. It should not be a requirement for a
Range in my opinion. I am making the assumption that no-one will be
particularly upset about loosening a constraint. Anyone wishing to add
default construction as a requirement to an algorithm can of course do so by
combining the SinglePassRangeConcept etc. with a separate constraint.

>
> A change from "T a;" to "const T& a" in concepts.hpp would do the trick...
>
> Example:
> struct AbstractArraySlice
> {
> int* begin() const = 0;
> int* end() const = 0;
> };
>
> // FAILS:
> BOOST_CONCEPT_ASSERT((SinglePassRangeConcept<AbstractArraySlice>))
>
> Footnote:
> I often use abstract base classes to verify my algorithms do not use more
> functionality than necessary and it annoys me that I can't use the range
> concept check:
>

I'm sorry you have been annoyed. I hope that the response to your request
has been dealt with in a manner that will dissipate the frustration.

I have a number of other requests and updates to upload. I need to test
these on a few compilers before I can upload this change, I therefore
estimate that I will have an update in place by Sunday 26 April 2009 23:59
GMT. This will be uploaded in the Boost Vault in the Algorithm directory as
range_ex.zip.

The URI for range_ex.zip:
http://www.boostpro.com/vault/index.php?action=downloadfile&filename=range_ex.zip&directory=Algorithms&

Thank you again for your feedback.

Best wishes,
Neil Groves