Why must a Single Pass Range be default constructible?
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? 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: struct TestSum { const AbstractArraySlice& r; void Test() { CalculateSum(r); } }; template<class T> T CalculateSum(const T& range) <... function requires SinglePassRangeConcept ...> -- View this message in context: http://www.nabble.com/Why-must-a-Single-Pass-Range-be-default-constructible-... Sent from the Boost - Users mailing list archive at Nabble.com.
Hello, thank you for your comments. On Tue, Apr 21, 2009 at 4:31 PM, athor <thorena@gmail.com> 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
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