C++核心準則R.24: 使用std::weak_ptr打破share_ptrs造成的循環(huán)

R.24: Use?std::weak_ptr?to break cycles of?shared_ptrs

R.24:?使用std::weak_ptr打破share_ptrs造成的循環(huán)

Reason（原因）

shared_ptr's rely on use counting and the use count for a cyclic structure never goes to zero, so we need a mechanism to be able to destroy a cyclic structure.

shared_ptr依靠使用計數(shù)動作，而循環(huán)構(gòu)造（例如相互持有shared_ptr，譯者注）可能導(dǎo)致計數(shù)永遠不歸零，因此我們需要一種機制打破這種循環(huán)。

Example（示例）

#include 

class bar;

class foo
{
public:
  explicit foo(const std::shared_ptr& forward_reference)
    : forward_reference_(forward_reference)
  { }
private:
  std::shared_ptr forward_reference_;
};

class bar
{
public:
  explicit bar(const std::weak_ptr& back_reference)
    : back_reference_(back_reference)
  { }
  void do_something()
  {
    if (auto shared_back_reference = back_reference_.lock()) {
      // Use *shared_back_reference
    }
  }
private:
  std::weak_ptr back_reference_;
};

Note（注意）

??? (HS: A lot of people say "to break cycles", while I think "temporary shared ownership" is more to the point.) ???(BS: breaking cycles is what you must do; temporarily sharing ownership is how you do it. You could "temporarily share ownership" simply by using another?shared_ptr.

???(HS:很多人說“打破循環(huán)”，我卻覺得“暫時分享所有權(quán)”才是關(guān)鍵）???(BS:打破循環(huán)是必須做的事，臨時分享所有權(quán)是做這件事的方法。你可以簡單地使用另外一個shared_ptr“暫時分享所有權(quán)”。

Enforcement（實施建議）

??? probably impossible. If we could statically detect cycles, we wouldn't need?weak_ptr

??? 差不多不可能。如果你能靜態(tài)檢查到循環(huán)，我們將不需要weak_ptr。

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#r23-use-make_unique-to-make-unique_ptrs

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