我真的很想将一些 unique_ptr 从一个 std::set move 到另一个:

#include <memory>
#include <algorithm>
#include <set>

int main()
{
   std::set<std::unique_ptr<int>> a;
   std::set<std::unique_ptr<int>> b;

   a.insert({0, std::unique_ptr<int>(new int(42))});

   std::move(a.begin(), a.end(), std::inserter(b, b.end()));
}

但是，我在 CentOS 7 上的 GCC 4.8.5 显然不满意:

[root@localhost ~]# g++ test.cpp -std=c++11 -o test
In file included from /usr/include/c++/4.8.2/set:60:0,
                 from test.cpp:2:
/usr/include/c++/4.8.2/bits/stl_tree.h: In instantiation of ‘std::_Rb_tree_node<_Val>::_Rb_tree_node(_Args&& ...) [with _Args = {const std::unique_ptr<int, std::default_delete<int> >&}; _Val = std::unique_ptr<int>]’:
/usr/include/c++/4.8.2/ext/new_allocator.h:120:4:   required from ‘void __gnu_cxx::new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::_Rb_tree_node<std::unique_ptr<int> >; _Args = {const std::unique_ptr<int, std::default_delete<int> >&}; _Tp = std::_Rb_tree_node<std::unique_ptr<int> >]’
/usr/include/c++/4.8.2/bits/alloc_traits.h:254:4:   required from ‘static typename std::enable_if<std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::value, void>::type std::allocator_traits<_Alloc>::_S_construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = std::_Rb_tree_node<std::unique_ptr<int> >; _Args = {const std::unique_ptr<int, std::default_delete<int> >&}; _Alloc = std::allocator<std::_Rb_tree_node<std::unique_ptr<int> > >; typename std::enable_if<std::allocator_traits<_Alloc>::__construct_helper<_Tp, _Args>::value, void>::type = void]’
/usr/include/c++/4.8.2/bits/alloc_traits.h:393:57:   required from ‘static decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) std::allocator_traits<_Alloc>::construct(_Alloc&, _Tp*, _Args&& ...) [with _Tp = std::_Rb_tree_node<std::unique_ptr<int> >; _Args = {const std::unique_ptr<int, std::default_delete<int> >&}; _Alloc = std::allocator<std::_Rb_tree_node<std::unique_ptr<int> > >; decltype (_S_construct(__a, __p, (forward<_Args>)(std::allocator_traits::construct::__args)...)) = <type error>]’
/usr/include/c++/4.8.2/bits/stl_tree.h:408:36:   required from ‘std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {const std::unique_ptr<int, std::default_delete<int> >&}; _Key = std::unique_ptr<int>; _Val = std::unique_ptr<int>; _KeyOfValue = std::_Identity<std::unique_ptr<int> >; _Compare = std::less<std::unique_ptr<int> >; _Alloc = std::allocator<std::unique_ptr<int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type = std::_Rb_tree_node<std::unique_ptr<int> >*]’
/usr/include/c++/4.8.2/bits/stl_tree.h:1023:66:   required from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Base_ptr, std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Base_ptr, _Arg&&) [with _Arg = const std::unique_ptr<int>&; _Key = std::unique_ptr<int>; _Val = std::unique_ptr<int>; _KeyOfValue = std::_Identity<std::unique_ptr<int> >; _Compare = std::less<std::unique_ptr<int> >; _Alloc = std::allocator<std::unique_ptr<int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::unique_ptr<int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Base_ptr = std::_Rb_tree_node_base*]’
/usr/include/c++/4.8.2/bits/stl_tree.h:1482:33:   required from ‘std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique_(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator, _Arg&&) [with _Arg = const std::unique_ptr<int>&; _Key = std::unique_ptr<int>; _Val = std::unique_ptr<int>; _KeyOfValue = std::_Identity<std::unique_ptr<int> >; _Compare = std::less<std::unique_ptr<int> >; _Alloc = std::allocator<std::unique_ptr<int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::unique_ptr<int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::unique_ptr<int> >]’
/usr/include/c++/4.8.2/bits/stl_tree.h:1722:37:   required from ‘void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(_II, _II) [with _InputIterator = const std::unique_ptr<int>*; _Key = std::unique_ptr<int>; _Val = std::unique_ptr<int>; _KeyOfValue = std::_Identity<std::unique_ptr<int> >; _Compare = std::less<std::unique_ptr<int> >; _Alloc = std::allocator<std::unique_ptr<int> >]’
/usr/include/c++/4.8.2/bits/stl_set.h:518:4:   required from ‘void std::set<_Key, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _InputIterator = const std::unique_ptr<int>*; _Key = std::unique_ptr<int>; _Compare = std::less<std::unique_ptr<int> >; _Alloc = std::allocator<std::unique_ptr<int> >]’
/usr/include/c++/4.8.2/bits/stl_set.h:530:9:   required from ‘void std::set<_Key, _Compare, _Alloc>::insert(std::initializer_list<_Tp>) [with _Key = std::unique_ptr<int>; _Compare = std::less<std::unique_ptr<int> >; _Alloc = std::allocator<std::unique_ptr<int> >]’
test.cpp:9:49:   required from here
/usr/include/c++/4.8.2/bits/stl_tree.h:140:49: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]’
    _M_value_field(std::forward<_Args>(__args)...) { }
                                                 ^
In file included from /usr/include/c++/4.8.2/memory:81:0,
                 from test.cpp:1:
/usr/include/c++/4.8.2/bits/unique_ptr.h:273:7: error: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^

我需要做什么才能完成这项工作？

最佳答案

在 C++14 中基本上不能这样做，因为集合的元素是 const。由于 move 是一种修改操作，您需要一些方法来获得对元素的非 const 访问，而根本没有办法做到这一点。

但是您将能够在 C++17 中使用新的 merge() 执行此操作成员函数:

b.merge(std::move(a));

类似地，将有一个 extract() 成员函数，它将删除并为您提供对单个节点的非 const 访问。

当然，你总是可以使用这样的类型(h/t by T.C. ):

struct Hack {
    mutable std::unique_ptr<T> p;
    T* raw_ptr;

    bool operator<(Hack const& h) const {
        // implemented in terms of the raw ptr
        // not the unique ptr
    }
};

从 p move 现在是安全的(它是可变的)，只要您只是 move (而不是 reset())并实现根据原始指针排序，那么您也应该保留集合排序。您现在必须存储两个指针，但这应该避免 C++11 中的 UB。

关于c++ - 为什么我不能 std::move std::unique_ptrs between std::sets？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/38679108/