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）
<code>#include <memory>
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_;
};/<memory>/<code>
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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