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Subject: Re: [boost] [contract] concepts: pseudo-signatures vs. usage patterns
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-10-14 03:59:28

On Sun, Oct 14, 2012 at 12:23 AM, Andrzej Krzemienski <akrzemi1_at_[hidden]>wrote:

> Can you help me understand one thing about pseudo-signatures? If I have the
> following concept:
> concept MyIter<typename It>
> {
> It& operator++(It&);
> bool It::is_valid();
> }
> Does this say preincrement returns exactly reference to It or only
> something convertible to It? If it is the latter, it would mean that the
> concept model where pre-increment only returns something convertible to It&
> satisfies the concept, but makes the following usage invalid:
> template <MyIter It>
> void test_next(It i)
> {
> return (++it).is_valid();
> }
> Or am I wrong?

My understanding, based only on reading this thread, is that, since It is
declared as a model of MyIter (I'm not sure what the correct terminology is
to express the relationship between It and MyIter within the scope of a
"template <MyIter It>" declaration, but that's what I mean by "is declared
as a model of"), ++it in the above context refers to the
(pseudo-?)signature declared in the MyIter concept definition (hence has
return type It&). The operator++ within the MyIter concept definition
implicitly (by default?) uses the operator++ of It, plus it adds a
(implicit) conversion of the result to an It&, if necessary.

So I would think the body of test_next would be entirely valid (no pun
intended)...modulo the attempted bool -> void conversion :)

Aside: This discussion, of which I've only been a casual observer, has
definitely been interesting.

- Jeff

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