Re: std::set<> and predicates

Rune Allnor <>
Mon, 5 Oct 2009 03:35:20 -0700 (PDT)
On 5 Okt, 09:46, James Kanze <> wrote:

On Oct 5, 4:55 am, Rune Allnor <> 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

and if it did use predicate, the above expression could never be

As I understand std::set and its predicates, the ordering of the
elements in the set is such that

less(*i,*j) == true

if i and j are iterators and initailized as above. If you are
right, I would have missed something very fundamental. Could you
elaborate, please?

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

Actually, in my code the inheritance is

class p : public std::less<size_t> {

So how should this be done?


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