为什么 make_move_iterator() 不能与 back_inserter() 一起使用？

Question

I'm trying to compile simple piece of code under C++17 standard:

#include <algorithm>
#include <iterator>
#include <vector>

using namespace std;

int main() {
    vector<int> numbers = {6, 1, 3, 9, 1, 9, 8, 12, 1};
    vector<int> res;

    copy(begin(numbers), end(numbers), make_move_iterator(back_inserter(res)));

    return 0;
}

I get an error on make_move_iterator(back_inserter(res)) .

Beginning of error is (excluding senseless (in my opinion) subsequent information):

In file included from C:/Users/Name/mingw/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_algobase.h:67,
                 from C:/Users/Name/mingw/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/algorithm:61,
                 from C:\Users\Name\CLionProjects\coursera_cpp_course3\week5\c3_w5_t6_merge_sort_3\c3_w5_t6_merge_sort_3.cpp:2:
C:/Users/Name/mingw/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_iterator.h: In instantiation of 'class std::move_iterator<std::back_insert_iterator<std::vector<int> > >':
C:\Users\Name\CLionProjects\coursera_cpp_course3\week5\c3_w5_t6_merge_sort_3\c3_w5_t6_merge_sort_3.cpp:49:77:   required from here
C:/Users/Name/mingw/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_iterator.h:1023:24: error: forming reference to void
     __base_ref>::type  reference;
                        ^~~~~~~~~

I've read documentation on make_move_iterator() , and haven't seen any preconditions on use of it here.

Why does it happen? Thank you for your suggestions.

Conditions under which I try to compile:

• Windows 10

• MinGW 8.1.0

• CMake 3.17.1

Update: I understood my fault. Here should be:

copy(make_move_iterator(begin(numbers)), make_move_iterator(end(numbers)), back_inserter(res));

Answer 1

std::make_move_iterator contructs an std::move_iterator , which is suppoed to work with an InputIterator , ie to move the values from the iterator instead of copying them.

std::move_iterator is an iterator adaptor which behaves exactly like the underlying iterator ( which must be at least an LegacyInputIterator ), except that dereferencing converts the value returned by the underlying iterator into an rvalue. If this iterator is used as an input iterator, the effect is that the values are moved from, rather than copied from .

Answer 2

The document you linked to clearly says:

Parameters

i - input iterator to be converted to move iterator

std::back_insert_iterator is not an input iterator , it is an output iterator . The value_type of std::back_insert_iterator is void , which is what the error message is complaining about:

forming reference to void