我想要一个 std::vector<std::chrono::milliseconds> 的排序 View 但我不想修改原始容器。 std::reference_wrapper看起来很适合这个，它对整数 vector 也适用。

我创建了这个小例子:

#include <chrono>
#include <vector>
#include <iostream>
#include <algorithm>
#include <functional>

int main()
{
    std::vector<int> numbers{1, 42, 3, 9, 5};
    std::vector<std::reference_wrapper<int>> sorted_numbers(numbers.begin(), numbers.end());
    std::sort(sorted_numbers.begin(), sorted_numbers.end());

    std::cout << "Numbers: ";
    for (const auto& n : numbers)
        std::cout << n << ' ';
    std::cout << '\n';

    std::cout << "Sorted numbers: ";
    for (const auto& n : sorted_numbers)
        std::cout << n << ' ';
    std::cout << '\n';

    std::cout << "Numbers: ";
    for (const auto& n : numbers)
        std::cout << n << ' ';
    std::cout << '\n';

    std::vector<std::chrono::milliseconds> durations{std::chrono::milliseconds{1},
                                                     std::chrono::milliseconds{42},
                                                     std::chrono::milliseconds{3},
                                                     std::chrono::milliseconds{9},
                                                     std::chrono::milliseconds{5}};
    std::vector<std::reference_wrapper<std::chrono::milliseconds>>
        sorted_durations(durations.begin(), durations.end());
    // std::sort(sorted_durations.begin(), sorted_durations.end());

    std::cout << "Durations: ";
    for (const auto& d : durations)
        std::cout << d.count() << ' ';
    std::cout << '\n';

    std::cout << "Sorted durations: ";
    for (const auto& d : sorted_durations)
        std::cout << d.get().count() << ' ';
    std::cout << '\n';

    std::cout << "Durations: ";
    for (const auto& d : durations)
        std::cout << d.count() << ' ';
    std::cout << '\n';
}

这会产生预期的输出(除了 sorted_durations 当然没有排序，因为它被注释掉了):

Numbers: 1 42 3 9 5 
Sorted numbers: 1 3 5 9 42 
Numbers: 1 42 3 9 5 
Durations: 1 42 3 9 5 
Sorted durations: 1 42 3 9 5 
Durations: 1 42 3 9 5 

As you can see, the original vector of integers numbers is unchanged by the sort operation done on sorted_numbers - that's exactly what I want for the sorted_durations vector as well. But when I uncomment that line my compiler gets very cross with me and I must admit that I can't figure out what it is trying to tell me. My compiler is clang++ version 3.8 and I build the example program like so:

clang++ -std=c++11 test.cc

And here's the error output I get:

In file included from test.cc:4:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/algorithm:62:
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:1935:11: error: invalid operands to binary expression
      ('std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >' and 'std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >')
        if (*__i < *__first)
            ~~~~ ^ ~~~~~~~~
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:5308:12: note: in instantiation of function template specialization
      'std::__heap_select<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >
      >' requested here
      std::__heap_select(__first, __middle, __last);
           ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:2310:24: note: in instantiation of function template specialization
      'std::partial_sort<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >
      >' requested here
              _GLIBCXX_STD_A::partial_sort(__first, __last, __last);
                              ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:5460:9: note: in instantiation of function template specialization
      'std::__introsort_loop<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >,
      long>' requested here
          std::__introsort_loop(__first, __last,
               ^
test.cc:35:10: note: in instantiation of function template specialization 'std::sort<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >
      *, std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > >
      > >' requested here
    std::sort(sorted_durations.begin(), sorted_durations.end());
         ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/chrono:488:7: note: candidate template ignored: could not match 'duration' against 'reference_wrapper'
      operator<(const duration<_Rep1, _Period1>& __lhs,
      ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/chrono:667:7: note: candidate template ignored: could not match 'time_point' against 'reference_wrapper'
      operator<(const time_point<_Clock, _Dur1>& __lhs,
      ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_pair.h:220:5: note: candidate template ignored: could not match 'pair' against 'reference_wrapper'
    operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:297:5: note: candidate template ignored: could not match 'reverse_iterator' against
      'reference_wrapper'
    operator<(const reverse_iterator<_Iterator>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:347:5: note: candidate template ignored: could not match 'reverse_iterator' against
      'reference_wrapper'
    operator<(const reverse_iterator<_IteratorL>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:1055:5: note: candidate template ignored: could not match 'move_iterator' against 'reference_wrapper'
    operator<(const move_iterator<_IteratorL>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:1061:5: note: candidate template ignored: could not match 'move_iterator' against 'reference_wrapper'
    operator<(const move_iterator<_Iterator>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_vector.h:1421:5: note: candidate template ignored: could not match 'vector' against 'reference_wrapper'
    operator<(const vector<_Tp, _Alloc>& __x, const vector<_Tp, _Alloc>& __y)
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/basic_string.h:2569:5: note: candidate template ignored: could not match 'basic_string' against 'reference_wrapper'
    operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/basic_string.h:2581:5: note: candidate template ignored: could not match 'basic_string' against 'reference_wrapper'
    operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/basic_string.h:2593:5: note: candidate template ignored: could not match 'const _CharT *' against
      'std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >'
    operator<(const _CharT* __lhs,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/array:238:5: note: candidate template ignored: could not match 'array' against 'reference_wrapper'
    operator<(const array<_Tp, _Nm>& __a, const array<_Tp, _Nm>& __b)
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/tuple:824:5: note: candidate template ignored: could not match 'tuple' against 'reference_wrapper'
    operator<(const tuple<_TElements...>& __t,
    ^
In file included from test.cc:4:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/algorithm:62:
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:61:
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_heap.h:235:35: error: invalid operands to binary expression ('std::reference_wrapper<std::chrono::duration<long,
      std::ratio<1, 1000> > >' and 'std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >')
          if (*(__first + __secondChild) < *(__first + (__secondChild - 1)))
              ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_heap.h:407:9: note: in instantiation of function template specialization
      'std::__adjust_heap<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >,
      long, std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > >' requested here
          std::__adjust_heap(__first, __parent, __len, _GLIBCXX_MOVE(__value));
               ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:1933:12: note: in instantiation of function template specialization
      'std::make_heap<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >
      >' requested here
      std::make_heap(__first, __middle);
           ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:5308:12: note: in instantiation of function template specialization
      'std::__heap_select<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >
      >' requested here
      std::__heap_select(__first, __middle, __last);
           ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:2310:24: note: in instantiation of function template specialization
      'std::partial_sort<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >
      >' requested here
              _GLIBCXX_STD_A::partial_sort(__first, __last, __last);
                              ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_algo.h:5460:9: note: in instantiation of function template specialization
      'std::__introsort_loop<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > *,
      std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > > >,
      long>' requested here
          std::__introsort_loop(__first, __last,
               ^
test.cc:35:10: note: in instantiation of function template specialization 'std::sort<__gnu_cxx::__normal_iterator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >
      *, std::vector<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >, std::allocator<std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > > > >
      > >' requested here
    std::sort(sorted_durations.begin(), sorted_durations.end());
         ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/chrono:488:7: note: candidate template ignored: could not match 'duration' against 'reference_wrapper'
      operator<(const duration<_Rep1, _Period1>& __lhs,
      ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/chrono:667:7: note: candidate template ignored: could not match 'time_point' against 'reference_wrapper'
      operator<(const time_point<_Clock, _Dur1>& __lhs,
      ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_pair.h:220:5: note: candidate template ignored: could not match 'pair' against 'reference_wrapper'
    operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:297:5: note: candidate template ignored: could not match 'reverse_iterator' against
      'reference_wrapper'
    operator<(const reverse_iterator<_Iterator>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:347:5: note: candidate template ignored: could not match 'reverse_iterator' against
      'reference_wrapper'
    operator<(const reverse_iterator<_IteratorL>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:1055:5: note: candidate template ignored: could not match 'move_iterator' against 'reference_wrapper'
    operator<(const move_iterator<_IteratorL>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_iterator.h:1061:5: note: candidate template ignored: could not match 'move_iterator' against 'reference_wrapper'
    operator<(const move_iterator<_Iterator>& __x,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/stl_vector.h:1421:5: note: candidate template ignored: could not match 'vector' against 'reference_wrapper'
    operator<(const vector<_Tp, _Alloc>& __x, const vector<_Tp, _Alloc>& __y)
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/basic_string.h:2569:5: note: candidate template ignored: could not match 'basic_string' against 'reference_wrapper'
    operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/basic_string.h:2581:5: note: candidate template ignored: could not match 'basic_string' against 'reference_wrapper'
    operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/bits/basic_string.h:2593:5: note: candidate template ignored: could not match 'const _CharT *' against
      'std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >'
    operator<(const _CharT* __lhs,
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/array:238:5: note: candidate template ignored: could not match 'array' against 'reference_wrapper'
    operator<(const array<_Tp, _Nm>& __a, const array<_Tp, _Nm>& __b)
    ^
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/tuple:824:5: note: candidate template ignored: could not match 'tuple' against 'reference_wrapper'
    operator<(const tuple<_TElements...>& __t,
    ^

它实际上持续了一段时间，但 stackoverflow 不允许我发布错误的所有 55000 个字符。

任何人都可以向我解释我做错了什么或者为什么这根本不可能(如果是这样的话)？

最佳答案

第一个错误似乎很明显:

In file included from [...]/algorithm:62:
[...]/bits/stl_algo.h:1935:11: error: invalid operands to binary expression
      ('std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >' and 'std::reference_wrapper<std::chrono::duration<long, std::ratio<1, 1000> > >')
        if (*__i < *__first)
            ~~~~ ^ ~~~~~~~~

您不能使用 <比较两个 reference_wrapper<duration<...>>对象。

转换为duration<...>不会发生，因为(如上面的评论所说)operator<对于 duration是一个函数模板，它的参数不能从 reference_wrapper<duration<...>> 推导出来.

如果你传递了一个 std::less<std::chrono::milliseconds> 的实例至 std::sort那么这将导致包装器转换为持续时间类型，并进行正确比较。

 std::sort(sorted_durations.begin(), sorted_durations.end(), std::less<std::chrono::milliseconds>{});

这基本上表示您希望通过将对象比较为 milliseconds 来对对象进行排序不是reference_wrapper<milliseconds> .

关于c++ - 为什么我不能对 reference_wrapper<std::chrono::milliseconds> 的 vector 进行排序？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36356166/