从std :: set提取仅移动类型 [英] Extracting move only type from std::set

问题描述

我有一个std::set<std::unique_ptr<T>>，我想将其移动到std::vector<std::unique_ptr<T>>

I have a std::set<std::unique_ptr<T>> and I'd like to move it to a std::vector<std::unique_ptr<T>>

#include <set>
#include <vector>
#include <memory>

class C {};

int main()
{
  std::set<std::unique_ptr<const C>> s;
  std::vector<std::unique_ptr<const C>> v;
  std::move(s.begin(), s.end(), std::back_inserter(v));
}

这在VS2017上出现以下错误:

This gives the following error on VS2017:

错误C2280:'std :: unique_ptr> :: unique_ptr(const std :: unique_ptr< _Ty，std :: default_delete< _Ty >>&)':试图引用已删除的函数

error C2280: 'std::unique_ptr>::unique_ptr(const std::unique_ptr<_Ty,std::default_delete<_Ty>> &)': attempting to reference a deleted function

我们不能将迭代器从std::set移至非常量变量吗?有什么办法可以解决这个问题?

Can't we make move iterators to non-const variables from a std::set? What could be a viable solution to this problem?

推荐答案

为了从集合中提取仅移动元素，唯一的可能性是使用

In order to extract move-only elements from a set, the only possibility is to use the extract method, which was added in C++17:

while (!s.empty())
    v.emplace_back(std::move(s.extract(s.begin()).value()));

如果您不能使用C ++ 17，则仅当您确保其在所施加的顺序中保持相同的位置时，才允许修改集合的元素(例如，使用mutable). -也就是说，只要与集合中的所有其他成员相比，它在您的比较器下具有相同的结果.为此，您可以提供一个比较器，该比较器在非空之前对空的唯一指针进行排序(请注意，标准不保证这样做)，并在修改后立即擦除修改后的元素:

If you cannot use C++17, it is permissible to modify an element of a set (e.g. using mutable) only if you ensure that it retains the same position in the imposed ordering - that is, as long as it has the same result under your comparator when compared to all other members of the set. You can do this by providing a comparator that orders empty unique pointers before non-empty (note that the standard does not guarantee this) and erasing the modified element immediately after modifying it:

template<class T> struct MutableWrapper { mutable T value; };
template<class T> struct MutableWrapperCompare {
  bool operator()(MutableWrapper<T> const& lhs, MutableWrapper<T> const& rhs) {
    return lhs.value && rhs.value ? lhs.value < rhs.value : rhs.value;
  }
};

int main()
{
  std::set<MutableWrapper<std::unique_ptr<const C>>, MutableWrapperCompare<std::unique_ptr<const C>>> s;
  std::vector<std::unique_ptr<const C>> v;
  while (!s.empty())
  {
    v.emplace_back(std::move(s.begin()->value));
    s.erase(s.begin());
  }
}

然而，这是非常丑陋和危险的；您最好使用 Boost.Container ，它具有C ++ 17提取方法(自1.62.0起 ；它没有记录，但这只是一个疏忽，请注意相应的extract方法记录在map和multimap中.)

This is however rather ugly and dangerous; you would be better off using boost::container::set from Boost.Container, which has the C++17 extract method (since 1.62.0; it was undocumented, but this is just an oversight, note the corresponding extract methods are documented for map and multimap).

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