Re: std::set<> and predicates
On 5 oct, 15:18, Michael Doubez <michael.dou...@free.fr> wrote:
On 5 oct, 12:35, Rune Allnor <all...@tele.ntnu.no> wrote:
On 5 Okt, 09:46, James Kanze <james.ka...@gmail.com> wrote:
On Oct 5, 4:55 am, Rune Allnor <all...@tele.ntnu.no> wrote:
I am a bit confused about std::set<> and associated
predicates. I have a std::set in my application where the
predicate works as expected when tested in isolation, but
where the items inserted in the set don't match the predicate,
that is, the test
class predicate : public std::less {/*...*/};
predicate p;
std::set<size_t> s(p);
// Insert elements
std::set<size_t>::iterator i = s.begin();
std::set<size_t>::iterator j = i; j++;
bool b = p(*i,*j); // Should be true
Knowing nothing about p, it's impossible to say much. But two
things are obvious: as declared above, your set doesn't use
predicate as an ordering function, it uses std::less<size_t>;
Why is that?
According to Josuttis' table 6.20, the constructor std::set<>::set()
with the predicate as argument should use the predicate as ordering
criterion.
set<> constructor keeps a copy of the Comparator. In your case, you
have slicing at the less<size_t> base of predicate.
I could not find the related section in the standard. I will post it
if/when I find it.
I could not locate it and the standard says that it is 'using the
specified comparison object' which is IMHO a bit misleading because we
could think it is using the underlying object. The only hint that I
found is that set<>::key_comp() and set<>::value_comp() returns an
object of type Compare and we could infer from that it must have a
value semantic. IMO it fits with the design of the STL.
The new standard proposal doesn't modify the wording.
However std::less<> doesn't have virtual functions and is not intended
as a base class. You don't even need to make it an explicit binary
predicate (defining result_type ... ) because 25.3 states that is will
be used as a function object.
--
Michael