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
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依靠使用計數動作，而循環構造（例如相互持有shared_ptr，譯者注）可能導致計數永遠不歸零，因此我們需要一種機制打破這種循環。
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:很多人說“打破循環”，我卻覺得“暫時分享所有權”纔是關鍵）???(BS:打破循環是必須做的事，臨時分享所有權是做這件事的方法。你可以簡單地使用另外一個shared_ptr“暫時分享所有權”。
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。如果你能靜態檢查到循環，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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