STL 旋转 unique_ptrs 的 const_iterators答案

【问题标题】：STL rotating const_iterators of unique_ptrsSTL 旋转 unique_ptrs 的 const_iterators
【发布时间】：2011-07-19 23:27:15
【问题描述】：

我在 const_iterator 上使用 std::rotate 时遇到了问题，而不是 unique_ptr 中间。

我试过了：

std::vector<std::unique_ptr<Object> >::const_iterator middle;
// middle is pointing at somewhere within vec defined elsewhere.
std::rotate(vec.begin(), middle, vec.end());

这导致调用rotate(...iterator, ...const_iterator &, ...iterator)没有匹配的函数

因此我尝试了这个： std::vector >::const_iterator first = vec.begin(); std::vector >::const_iterator last = vec.end(); std::rotate(first, middle, last);

然后它会给出如下所示的一堆错误消息。这是一个错误还是我犯了一些错误？

In file included from /usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/algorithm:63:0,
             from file.cpp:8:
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h: In function ‘void std::__rotate(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, std::random_access_iterator_tag) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’:
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1677:7:   instantiated from ‘void std::rotate(_FIter, _FIter, _FIter) [with _FIter = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
file.cpp:76:35:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h:207:7: error: deleted function ‘std::unique_ptr<_Tp, _Tp_Deleter>::unique_ptr(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1601:22: error: used here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1677:7:   instantiated from ‘void std::rotate(_FIter, _FIter, _FIter) [with _FIter = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
file.cpp:76:35:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1603:5: error: passing ‘const std::unique_ptr<Object>’ as ‘this’ argument of ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(std::unique_ptr<_Tp, _Tp_Deleter>&&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’ discards qualifiers
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1677:7:   instantiated from ‘void std::rotate(_FIter, _FIter, _FIter) [with _FIter = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
file.cpp:76:35:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h:207:7: error: deleted function ‘std::unique_ptr<_Tp, _Tp_Deleter>::unique_ptr(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1624:22: error: used here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1677:7:   instantiated from ‘void std::rotate(_FIter, _FIter, _FIter) [with _FIter = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
file.cpp:76:35:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1626:5: error: passing ‘const std::unique_ptr<Object>’ as ‘this’ argument of ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(std::unique_ptr<_Tp, _Tp_Deleter>&&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’ discards qualifiers
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_tree.h:62:0,
             from /usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/map:60,
             from dir/file3.hpp:4,
             from dir/file2.hpp:3,
             from file.hpp:3,
             from file.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h: In static member function ‘static void std::__iter_swap<_BoolType>::iter_swap(_ForwardIterator1, _ForwardIterator2) [with _ForwardIterator1 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >, _ForwardIterator2 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >, bool _BoolType = false]’:
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:138:7:   instantiated from ‘void std::iter_swap(_ForwardIterator1, _ForwardIterator2) [with _ForwardIterator1 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >, _ForwardIterator2 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1609:5:   instantiated from ‘void std::__rotate(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, std::random_access_iterator_tag) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1677:7:   instantiated from ‘void std::rotate(_FIter, _FIter, _FIter) [with _FIter = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
file.cpp:76:35:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h:207:7: error: deleted function ‘std::unique_ptr<_Tp, _Tp_Deleter>::unique_ptr(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:88:31: error: used here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:89:11: error: passing ‘const std::unique_ptr<Object>’ as ‘this’ argument of ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’ discards qualifiers
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h:208:19: error: deleted function ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:89:11: error: used here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:90:11: error: passing ‘const std::unique_ptr<Object>’ as ‘this’ argument of ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(std::unique_ptr<_Tp, _Tp_Deleter>&&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’ discards qualifiers
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h: In static member function ‘static _OI std::__copy_move<true, false, std::random_access_iterator_tag>::__copy_m(_II, _II, _OI) [with _II = const std::unique_ptr<Object>*, _OI = const std::unique_ptr<Object>*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:404:70:   instantiated from ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = true, _II = const std::unique_ptr<Object>*, _OI = const std::unique_ptr<Object>*]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:442:39:   instantiated from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = true, _II = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >, _OI = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:506:47:   instantiated from ‘_OI std::move(_II, _II, _OI) [with _II = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >, _OI = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1602:5:   instantiated from ‘void std::__rotate(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, std::random_access_iterator_tag) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1677:7:   instantiated from ‘void std::rotate(_FIter, _FIter, _FIter) [with _FIter = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
file.cpp:76:35:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:368:8: error: passing ‘const std::unique_ptr<Object>’ as ‘this’ argument of ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’ discards qualifiers
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h:208:19: error: deleted function ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:368:8: error: used here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h: In static member function ‘static _BI2 std::__copy_move_backward<true, false, std::random_access_iterator_tag>::__copy_move_b(_BI1, _BI1, _BI2) [with _BI1 = const std::unique_ptr<Object>*, _BI2 = const std::unique_ptr<Object>*]’:
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:601:18:   instantiated from ‘_BI2 std::__copy_move_backward_a(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = const std::unique_ptr<Object>*, _BI2 = const std::unique_ptr<Object>*]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:610:34:   instantiated from ‘_BI2 std::__copy_move_backward_a2(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >, _BI2 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:681:15:   instantiated from ‘_BI2 std::move_backward(_BI1, _BI1, _BI2) [with _BI1 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >, _BI2 = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1625:5:   instantiated from ‘void std::__rotate(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, std::random_access_iterator_tag) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algo.h:1677:7:   instantiated from ‘void std::rotate(_FIter, _FIter, _FIter) [with _FIter = __gnu_cxx::__normal_iterator<const std::unique_ptr<Object>*, std::vector<std::unique_ptr<Object> > >]’
file.cpp:76:35:   instantiated from here
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:566:6: error: passing ‘const std::unique_ptr<Object>’ as ‘this’ argument of ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’ discards qualifiers
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/unique_ptr.h:208:19: error: deleted function ‘std::unique_ptr<_Tp, _Tp_Deleter>& std::unique_ptr<_Tp, _Tp_Deleter>::operator=(const std::unique_ptr<_Tp, _Tp_Deleter>&) [with _Tp = Object, _Tp_Deleter = std::default_delete<Object>, std::unique_ptr<_Tp, _Tp_Deleter> = std::unique_ptr<Object>]’
/usr/lib/gcc/x86_64-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_algobase.h:566:6: error: used here
make: *** [file.o] Error 1

【问题讨论】：

标签： c++ iterator rotation unique-ptr const-iterator

【解决方案1】：

您不能将rotate 与const_iterators 一起使用，因为rotate 会更改指定范围内的元素。相反，您必须使用iterators。

【讨论】：

但是对象本身并没有改变，只是移动了，对吧？
@chemelnucfin：迭代器没有改变，但它们指向的内容是。请注意const iterator 和const_iterator 之间的区别，要旋转的迭代器实际上是通过副本传递的。
@tomasz，我尝试过对上述项目使用交换，但它似乎对 const_iterators 有效？唯一没有混合 const_iter 和 iter。 swap(players.begin(), middle) 没有，但是 const_iter i = vec.begin() => swap(i, middle) 有效。我没有测试实际结果，但至少编译过。
@chemelnucfin：swap 不将迭代器作为参数，而是将对象的引用作为交换对象。您刚刚交换了两个迭代器（保持内容不变）。就像const_iter tmp = i; i = middle; middle = tmp;。可能不是你想要的。如果你需要交换内容，你必须调用swap(*i, *middle)，但是它们不能是const_iterator类型。
@chemelnucfin 也许你可以使用rotate_copy。

【解决方案2】：

如果cplusplus documentation 是trusted，那么rotate 本质上就是调用swap(T &, T &)，所以你不能使用const_iterators。

（我想有人可能会要求一个专门版本的rotate 用于在 const_iterators 上运行的列表。不过，我认为您可以相当容易地构建一个。）

【讨论】：