std::multiset 交集编译错误答案

【问题标题】：std::multiset intersection compilation errorstd::multiset 交集编译错误
【发布时间】：2021-06-15 04:11:36
【问题描述】：

我正在尝试使用 std::set_intersection 函数查找 2 个多组对象的交集。

#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;
}

当我尝试编译它时，我得到一页我无法正确理解的详细错误。尽管我确实了解一些基本概念，但我仍然是 C++ 的新手。下面是生成的错误日志，

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).

任何帮助将不胜感激。谢谢。

更新： 从错误日志中，我看到需要为 std::set_intersection 传递另一个可选参数以用于自定义比较器。

/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]’

_Compare 参数看起来是默认的 __gnu_cxx::__ops::_Iter_less_iter。我想使用我在那里制作的比较器类（cmp_interval），但它不接受类。

【问题讨论】：

你应该超载operator <。
@PaulMcKenzie 您只需要为默认比较器执行此操作。他们提供了一个自定义比较器。
@Jamāl -- 我知道你使用了 operator()，但只有在将项目插入集合时才会这样做。它与set_intersection 调用无关。请参阅弗朗索瓦先前的评论。你需要告诉set_intersection如何比较两个项目。
添加 cmp_interval{} 作为另一个参数，它应该可以解决所有问题。
@Jamāl std::set_intersection 需要知道如何比较元素。由于您没有重载operator<，而是使用了比较器，因此您需要将该比较器提供给std::set_intersection，否则它不知道如何检查哪些元素相交。

标签： c++ c++17 std multiset set-intersection

【解决方案1】：

std::multiset<Interval, cmp_interval> I3;

在这里，您指定比较器的类型，作为类型模板参数。实际的比较器对象是在multiset 的构造函数中创建的。现在您想将类似的 object 传递给 std::set_intersection 函数。使用：

std::set_intersection(I1.begin(), I1.end(), 
                      I2.begin(), I2.end(), 
                      std::inserter(I3, I3.begin()),
                      cmp_interval());

cmp_interval() 中的空括号表示：未命名的默认构造对象。从 C++11 开始，您也可以使用 cmp_interval{}。

【讨论】：