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c++ - std::multiset 交集编译错误

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。谢谢你。

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

/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。

标签： c++c++17stdmultisetset-intersection

解决方案

std::multiset<Interval, cmp_interval> I3;

在这里，您将比较器的类型指定为类型模板参数。实际的比较器对象是在multiset. 现在您想将类似的对象传递给std::set_intersection函数。利用：

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

mean中的空括号 cmp_interval()：一个未命名的默认构造对象。cmp_interval{}从 C++11 开始，您也可以使用,。

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