Perché non riesco a ordinare un vettore di reference_wrapper <std :: chrono :: milliseconds>?

Question

Desidero una visualizzazione ordinata di std::vector<std::chrono::milliseconds> ma non desidero modificare il contenitore originale. std::reference_wrapper sembra perfetto per questo e funziona perfettamente per un vettore di numeri interi.

Ho creato questo piccolo esempio:

#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'; 
} 

che produce l'uscita prevista (salvo che sorted_durations non è ordinato, naturalmente, dal momento che è commentato out):

 
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 

Come si può vedere, il vettore originale di interi numbers è invariato dall'operazione di ordinamento effettuata su sorted_numbers - è esattamente quello che voglio anche per il vettore sorted_durations. Ma quando disapprovo quella riga il mio compilatore diventa molto arrabbiato con me e devo ammettere che non riesco a capire cosa stia cercando di dirmi. mio compilatore è clang ++ versione 3.8 e costruire il programma di esempio in questo modo:

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

Ed ecco l'output errore che ottengo:

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, 
    ^

E 'in realtà va avanti per un po' di più, ma StackOverflow non lo farà fammi postare tutti i 55000 caratteri dell'errore.

Qualcuno può spiegarmi che cosa sto sbagliando o perché semplicemente non è possibile (se è così)?

Answer 1

Il primo errore sembra abbastanza chiaro:

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) 
      ~~~~^~~~~~~~~ 

Non è possibile utilizzare < per confrontare due reference_wrapper<duration<...>> oggetti.

La conversione al duration<...> non avviene perché (come dice un commento sopra) la operator< per duration è un modello di funzione ei suoi argomenti non può essere dedotta da reference_wrapper<duration<...>>.

Se si passava un'istanza di std::less<std::chrono::milliseconds> a std::sort, ciò avrebbe causato la conversione dei wrapper nei tipi di durata e il confronto corretto.

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

Questo dice in sostanza che si desidera ordinare gli oggetti confrontandoli come milliseconds non reference_wrapper<milliseconds>.

Answer 2

Per @ suggerimento di Niall, è possibile utilizzare un lambda come comparatore per la sort():

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(); });