C++核心准则R.24: 使用std::weak_ptr打破share_ptrs造成的循环

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

R.24: 使用std::weak_ptr打破share_ptrs造成的循环

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依靠使用计数动作，而循环构造（例如相互持有shared_ptr，译者注）可能导致计数永远不归零，因此我们需要一种机制打破这种循环。

Example（示例）

#include <memory>

class bar;

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

class bar
{
public:
  explicit bar(const std::weak_ptr<foo>& 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<foo> 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:很多人说“打破循环”，我却觉得“暂时分享所有权”才是关键）???(BS:打破循环是必须做的事，临时分享所有权是做这件事的方法。你可以简单地使用另外一个shared_ptr“暂时分享所有权”。

Enforcement（实施建议）

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

??? 差不多不可能。如果你能静态检查到循环，我们将不需要weak_ptr。

原文链接

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

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