为什么我不能对 reference_wrapper<std：：chrono：：毫秒>的向量进行排序？
Why can't I sort a vector of reference_wrapper<std::chrono::milliseconds>?
我想要一个std::vector<std::chrono::milliseconds>的排序视图，但我不想修改原始容器。std::reference_wrapper似乎非常适合这一点，它对整数向量也很好。
我创建了一个小示例：
#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';
}
它产生了预期的输出（除了sorted_durations当然没有排序，因为它被注释掉了）：
数字：1 42 3 9 5排序编号：1 3 5 9 42数字：1 42 3 9 5工期：1 42 3 9 5排序持续时间：1 42 3 9 5工期：1 42 3 9 5
正如你所看到的，整数numbers的原始向量在sorted_numbers上执行的排序操作中没有改变——这正是我想要的sorted_durations向量。但当我取消注释那一行时，我的编译器会非常生气，我必须承认，我不明白它试图告诉我什么。我的编译器是clang++版本3.8，我构建了如下示例程序：
clang++-std=c++11test.cc
这是我得到的错误输出：
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,
^
事实上，它会持续更长的时间，但stackoverflow不会让我发布所有55000个字符的错误。
有人能向我解释我做错了什么吗？或者为什么这根本不可能（如果是这样的话）？
第一个错误似乎很清楚：
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)
~~~~ ^ ~~~~~~~~
不能使用<来比较两个reference_wrapper<duration<...>>对象。
之所以没有转换为duration<...>，是因为（正如上面的注释所说）duration的operator<是一个函数模板，其参数无法从reference_wrapper<duration<...>>推导出来。
如果您将std::less<std::chrono::milliseconds>的实例传递给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>{});
这基本上说明您希望通过将对象比较为milliseconds而不是reference_wrapper<milliseconds>来对它们进行排序。
根据@Niall的建议，您可以使用lambda作为sort():的比较器
std::sort(sorted_durations.begin(), sorted_durations.end(),
[](const std::reference_wrapper<std::chrono::milliseconds> &a,
const std::reference_wrapper<std::chrono::milliseconds> &b)
-> bool { return a.get() < b.get(); } );
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