Does std::set.find(element) use the == operator from the class to compare the elements?

Question

Say I have

std::set<classtype> set;
class classtype {
    bool operator==(const classtype& ct) {
        //..
    } 
};
//..
std::set<classtype>::iterator it = set.find(element);

Find does use the == operator from the class correct?

Also my reference says it has log(n) worst case runtime where n is the number of elements in the set. How is this realized internally? I understand that the key is that the elements in the set have a order (so insertion takes long to create that order), for integer sets it is clear what order means but for random classes not so much.

No, it's [`srd::less`](http://en.cppreference.com/w/cpp/container/set) actually applied as default. — πάντα ῥεῖ, Jun 24 '15 at 19:27
@πάνταῥεῖ `std`? Also, order is defined by your implementation of a less than operation. — scohe001, Jun 24 '15 at 19:34
@myelf: If you want to search using your `operator==`, then you can use `std::find`. But then the search is not taking advantage of the ordered nature of the set, and is O(n) instead of O(log n). — Benjamin Lindley, Jun 24 '15 at 19:48
Very hard to do with any speed. == tells you match or not, but not where in the set you are relative to where the target should be. — user4581301, Jun 24 '15 at 19:58

Vlad from Moscow · Accepted Answer · 2015-06-24T20:08:37.020

5

From the C++ Standard (23.2.4 Associative containers)

3 The phrase “equivalence of keys” means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false. For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value.

Member function find seeks a key according to the comparison object comp

If you did not specify explicitly the comparison object then the class uses by default standard functional object std::less that uses operator < within its operator function. So your class has to have the operator < defined.

If you want to use operator == for comparison values in the set then you can use standard algorithm std::find instead of the method find.

edited Jun 24 '15 at 20:08

answered Jun 24 '15 at 19:36

Vlad from Moscow

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can you give a tip what makes sense for a < operator for a std::vector? – myelf Jun 24 '15 at 19:51
1

@myelf: You could use the one that is already defined in the standard library, which does a lexicographical comparison. http://en.cppreference.com/w/cpp/container/vector/operator_cmp – Benjamin Lindley Jun 24 '15 at 19:52
Thanks. Unfortunately this does not make much sense for what I am trying to do. – myelf Jun 24 '15 at 20:01
Oh, thanks! When I use std::set::iterator it= std::find(std::begin(std::set), std::end(std::set), elementOfType) is this what I "want" to do? I guess the runtime of this is bad. – myelf Jun 24 '15 at 21:00
@myelf yes. It is a linear search with O( size() ). – Vlad from Moscow Jun 24 '15 at 21:02

Does std::set.find(element) use the == operator from the class to compare the elements?

1 Answers1