std::multiset intersection compilation error - c++

Question

I'm trying to find the intersection of 2 multisets of objects using std::set_intersection function.
#include <algorithm>
#include <set>
struct Interval
{
double bottom;
double top;
};
struct cmp_interval
{
bool operator()(Interval a, Interval b) const
{
return a.bottom < b.bottom;
}
}
int main()
{
std::multiset<Interval, cmp_interval> I1;
Interval i;
i.bottom = 20;
i.top = 30;
I1.insert(i);
std::multiset<Interval, cmp_interval> I2;
I2.insert(i);
std::multiset<Interval, cmp_interval> I3;
std::set_intersection(I1.begin(), I1.end(), I2.begin(), I2.end(), std::inserter(I3, I3.begin()));
return 0;
}
When I try to compile it, i get a page of verbose errors that I'm unable to properly understand. I'm still a novice in c++ although i do understand some fundamental concepts. Below is the error log generated,
Starting build...
/usr/bin/g++ -g /home/jamal/Documents/daa_assignment_1/measure.cpp -o /home/jamal/Documents/daa_assignment_1/measure
In file included from /usr/include/c++/9/bits/stl_algobase.h:71,
from /usr/include/c++/9/bits/char_traits.h:39,
from /usr/include/c++/9/ios:40,
from /usr/include/c++/9/ostream:38,
from /usr/include/c++/9/iostream:39,
from /home/jamal/Documents/daa_assignment_1/measure.cpp:1:
/usr/include/c++/9/bits/predefined_ops.h: In instantiation of ‘constexpr bool __gnu_cxx::__ops::_Iter_less_iter::operator()(_Iterator1, _Iterator2) const [with _Iterator1 = std::_Rb_tree_const_iterator<Interval>; _Iterator2 = std::_Rb_tree_const_iterator<Interval>]’:
/usr/include/c++/9/bits/stl_algo.h:5252:12: required from ‘_OutputIterator std::__set_intersection(_InputIterator1, _InputIterator1, _InputIterator2, _InputIterator2, _OutputIterator, _Compare) [with _InputIterator1 = std::_Rb_tree_const_iterator<Interval>; _InputIterator2 = std::_Rb_tree_const_iterator<Interval>; _OutputIterator = std::insert_iterator<std::multiset<Interval, cmp_interval> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]’
/usr/include/c++/9/bits/stl_algo.h:5307:48: required from ‘_OIter std::set_intersection(_IIter1, _IIter1, _IIter2, _IIter2, _OIter) [with _IIter1 = std::_Rb_tree_const_iterator<Interval>; _IIter2 = std::_Rb_tree_const_iterator<Interval>; _OIter = std::insert_iterator<std::multiset<Interval, cmp_interval> >]’
/home/jamal/Documents/daa_assignment_1/measure.cpp:338:100: required from here
/usr/include/c++/9/bits/predefined_ops.h:43:23: error: no match for ‘operator<’ (operand types are ‘const Interval’ and ‘const Interval’)
43 | { return *__it1 < *__it2; }
| ~~~~~~~^~~~~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:67,
from /usr/include/c++/9/bits/char_traits.h:39,
from /usr/include/c++/9/ios:40,
from /usr/include/c++/9/ostream:38,
from /usr/include/c++/9/iostream:39,
from /home/jamal/Documents/daa_assignment_1/measure.cpp:1:
/usr/include/c++/9/bits/stl_iterator.h:915:5: note: candidate: ‘template<class _IteratorL, class _IteratorR, class _Container> bool __gnu_cxx::operator<(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)’
915 | operator<(const __normal_iterator<_IteratorL, _Container>& __lhs,
| ^~~~~~~~
/usr/include/c++/9/bits/stl_iterator.h:915:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/9/bits/stl_algobase.h:71,
from /usr/include/c++/9/bits/char_traits.h:39,
from /usr/include/c++/9/ios:40,
from /usr/include/c++/9/ostream:38,
from /usr/include/c++/9/iostream:39,
from /home/jamal/Documents/daa_assignment_1/measure.cpp:1:
/usr/include/c++/9/bits/predefined_ops.h:43:23: note: ‘const Interval’ is not derived from ‘const __gnu_cxx::__normal_iterator<_IteratorL, _Container>’
43 | { return *__it1 < *__it2; }
| ~~~~~~~^~~~~~~~
In file included from /usr/include/c++/9/bits/stl_algobase.h:67,
from /usr/include/c++/9/bits/char_traits.h:39,
from /usr/include/c++/9/ios:40,
from /usr/include/c++/9/ostream:38,
from /usr/include/c++/9/iostream:39,
from /home/jamal/Documents/daa_assignment_1/measure.cpp:1:
/usr/include/c++/9/bits/stl_iterator.h:922:5: note: candidate: ‘template<class _Iterator, class _Container> bool __gnu_cxx::operator<(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)’
922 | operator<(const __normal_iterator<_Iterator, _Container>& __lhs,
| ^~~~~~~~
/usr/include/c++/9/bits/stl_iterator.h:922:5: note: template argument deduction/substitution failed:
In file included from /usr/include/c++/9/bits/stl_algobase.h:71,
from /usr/include/c++/9/bits/char_traits.h:39,
from /usr/include/c++/9/ios:40,
from /usr/include/c++/9/ostream:38,
from /usr/include/c++/9/iostream:39,
from /home/jamal/Documents/daa_assignment_1/measure.cpp:1:
/usr/include/c++/9/bits/predefined_ops.h:43:23: note: ‘const Interval’ is not derived from ‘const __gnu_cxx::__normal_iterator<_Iterator, _Container>’
43 | { return *__it1 < *__it2; }
| ~~~~~~~^~~~~~~~
Build finished with error(s).
The terminal process failed to launch (exit code: -1).
Any help would be appreciated. Thank you.
Update:
From the error log, I see that there's another optional argument that needs to be passed for std::set_intersection for custom comparator.
/usr/include/c++/9/bits/stl_algo.h:5252:12: required from ‘_OutputIterator std::__set_intersection(_InputIterator1, _InputIterator1, _InputIterator2, _InputIterator2, _OutputIterator, _Compare) [with _InputIterator1 = std::_Rb_tree_const_iterator<Interval>; _InputIterator2 = std::_Rb_tree_const_iterator<Interval>; _OutputIterator = std::insert_iterator<std::multiset<Interval, cmp_interval> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]’
The _Compare parameter looks to be by default
__gnu_cxx::__ops::_Iter_less_iter. I want to use comparator class that i have made(cmp_interval) there but it doesn't accept classes.

Answer 1

std::multiset<Interval, cmp_interval> I3;
Here, you specify the type of the comparator, as a type template argument. The actual comparator object is created inside the constructor of the multiset. Now you want to pass a similar object to the std::set_intersection function. Use:
std::set_intersection(I1.begin(), I1.end(),
I2.begin(), I2.end(),
std::inserter(I3, I3.begin()),
cmp_interval());
The empty parenthesis in cmp_interval() mean: an unnamed default-constructed object. You could also use cmp_interval{}, since C++11.