将键移出 std::map<>&& [英] Moving keys out of std::map<> &&

问题描述

我想让我们说 getKeys() 函数从 map 中获取不可复制的键:

I'd like to have let's say getKeys() function getting not-copiable keys out of a map:

class MyObj {
  // ... complex, abstract class...
};

struct Comparator { bool operator()(std::unique_ptr<MyObj> const &a, std::unique_ptr<MyObj> const &b); };

std::vector<std::unique_ptr<MyObj>> getKeys(std::map<std::unique_ptr<MyObj>, int, Comparator> &&map) {
  std::vector<std::unique_ptr<MyObj>> res;
  for (auto &it : map) {
    res.push_back(std::move(it.first));
  }
  return res;
}

但是它不起作用，因为it (.first) 中的键是const.任何提示如何解决它?注意:在我们的环境中，我不允许使用 C++17 函数 std::map::extract().

But it is not working because the key in it (.first) is const. Any tips how to solve it? Note: In our environment I'm not allowed to use C++17 function std::map::extract().

是否可以使用 const_cast 因为 map 无论如何都会被破坏?

Is it somehow ok to use const_cast because map will be destructed anyway?

res.push_back(std::move(const_cast<std::unique_ptr<MyObj> &>(it.first)));

我想避免克隆 MyObj.

我知道为什么不能修改 std::map 容器的键，但是对于在键修改后立即销毁的映射仍然不允许吗?

I know why keys of a std::map container cannot be modified but is it still disallowed for a map that is going to be destructed immediately after the key modification?

推荐答案

注意:在我们的环境中，我不允许使用 C++17 函数 std::map::extract().

耻辱 - 它是为了解决这个问题而引入的.

Shame - it was introduced to solve this problem.

是否可以使用 const_cast 因为地图无论如何都会被破坏?

Is it somehow ok to use const_cast because map will be destructed anyway?

没有

我想避免克隆 MyObj.

对不起；您至少需要克隆密钥.

Sorry; you'll need to clone the keys at least.

我知道为什么不能修改 std::map 容器的键，但是对于在键修改后立即销毁的映射仍然不允许吗?

I know why keys of a std::map container cannot be modified but is it still disallowed for a map that is going to be destructed immediately after the key modification?

是的.

地图的内部机制无法知道它的命运在等待.

The map's internal machinery has no way of knowing that its destiny awaits.

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