Re: std::set<> and predicates

"Francesco S. Carta" <>
Mon, 5 Oct 2009 04:29:58 -0700 (PDT)
On 5 Ott, 12:36, Rune Allnor <> wrote:

On 5 Okt, 06:29, "Alf P. Steinbach" <> wrote:

* Rune Allnor:

Are there any obvious pitfalls I have missed here?

Make sure that for any values A, B and C your predicate ensures that A<B && B<C
implies A<C, and that A<C implies !(C<A).

As I said, the predicate works as expected when tested
in isolation outside the std::set<>.

How do you tell that it is not working when used by set?

The following code, adapted from your OP, proves that it works:
#include <iostream>
#include <set>

class predicate : public std::less<size_t> {};

int main()
    predicate p;
    std::set<size_t> s(p);
    std::set<size_t>::iterator i = s.begin();
    std::set<size_t>::iterator j = i;
    std::cout << "*i == " << *i << std::endl;
    std::cout << "*j == " << *j << std::endl;
    if(p(*i,*j)) {
        std::cout << "true" << std::endl;
    } else {
        std::cout << "false" << std::endl;


*i == 10
*j == 20

Where exactly are you stuck in?

 Francesco S. Carta,
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