Re: std::set<> and predicates

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 5 Oct 2009 00:46:50 -0700 (PDT)

Message-ID:

<11db37d6-87d2-4ed4-bfb7-6b90bdaa690d@p15g2000vbl.googlegroups.com>

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>;
and if it did use predicate, the above expression could never be
true.

Also, even the extract of your code is illegal, since you can't
derive from std::less (which is a template, not a class).

[...]

Could somebody please provide links to useful information on
the template arguments for std::less<>?

There's only one, the type to be compared. What more
information do you need?

--
James Kanze