如何从STL容器中获取仅移动类型? [英] How to get a move-only type out of a STL container?
问题描述
让我们以std::unique_ptr<T>
的std::unordered_set
为例.我可以将集合中的元素移到其他地方吗?
Let's consider an std::unordered_set
of std::unique_ptr<T>
as an example. Can I move an element of the set elsewhere ?
#include <unordered_set>
#include <iostream>
#include <memory>
#include <vector>
int main()
{
std::unordered_set<std::unique_ptr<int>> mySet;
mySet.insert(std::make_unique<int>(1));
mySet.insert(std::make_unique<int>(2));
mySet.insert(std::make_unique<int>(3));
std::vector<std::unique_ptr<int>> myVector;
for (auto&& element : mySet)
{
std::cout << *element << std::endl;
//myVector.push_back(element); won't compile as you can only get a const ref to the key
}
}
我有一个非常实用的代码示例,我想这样做,但是简化为使用std::shared_ptr
.您知道另一种(更好的?)替代方法吗?
I have a very practical example of code where I would like to do this but am reduced to use a std::shared_ptr
. Do you know of another (better ?) alternative ?
推荐答案
在C ++ 03,C ++ 11和C ++ 14中,不是直接使用.您必须将类型更改为:
In C++03, C++11, and C++14, not directly. You'd have to change the type to be something like:
template <class T>
struct handle {
mutable std::unique_ptr<T> owning_ptr;
T* observing_ptr; // enforce that observing_ptr == owning_ptr.get() on construction
// define operator<, hash, etc. in terms of the observing ptr
};
使用此代码,您可以编写:
With this, you can write:
std::unordered_set<handle<int>> mySet;
// initialize as appropriate
for (auto& elem : mySet) {
myVector.push_back(std::move(elem.owning_ptr));
}
mySet.clear();
这仍然是定义明确的行为,因为我们不会弄乱任何容器内部结构-观察指针在clear()
的末尾仍然有效,只是现在myVector
拥有它.
This will still be well-defined behavior because we're not messing with any of the container internals - the observing pointer will still be valid through the end of the clear()
, just now myVector
owns it instead.
In C++17, we can do this directly and more simply with the help of extract()
:
for (auto it = mySet.begin(); it != mySet.end();
{
std::cout << **it << std::endl;
myVector.push_back(std::move(
mySet.extract(it++).value()));
}
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