`enable_shared_from_this` 有什么用处?-C/C++问题

What is the usefulness of `enable_shared_from_this`?(`enable_shared_from_this` 有什么用处?)

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问题描述

我在阅读 Boost.Asio 示例时遇到了 enable_shared_from_this，在阅读文档后，我仍然不知道如何正确使用它.有人可以给我一个例子和解释什么时候使用这个类是有意义的.

I ran across enable_shared_from_this while reading the Boost.Asio examples and after reading the documentation I am still lost for how this should correctly be used. Can someone please give me an example and explanation of when using this class makes sense.

推荐答案

当你只有 这个.没有它，您将无法获得 shared_ptr 到 this，除非您已经拥有一个成员.此示例来自 enable_shared_from_this 的 boost 文档:>


It enables you to get a valid shared_ptr instance to this, when all you have is this. Without it, you would have no way of getting a shared_ptr to this, unless you already had one as a member. This example from the boost documentation for enable_shared_from_this:
class Y: public enable_shared_from_this<Y>
{
public:

    shared_ptr<Y> f()
    {
        return shared_from_this();
    }
}

int main()
{
    shared_ptr<Y> p(new Y);
    shared_ptr<Y> q = p->f();
    assert(p == q);
    assert(!(p < q || q < p)); // p and q must share ownership
}

方法 f() 返回一个有效的 shared_ptr，即使它没有成员实例.请注意，您不能简单地执行此操作:
The method f() returns a valid shared_ptr, even though it had no member instance. Note that you cannot simply do this:
class Y: public enable_shared_from_this<Y>
{
public:

    shared_ptr<Y> f()
    {
        return shared_ptr<Y>(this);
    }
}

此返回的共享指针将具有与正确"引用计数不同的引用计数，并且其中之一将最终丢失并在删除对象时保持悬空引用.
The shared pointer that this returned will have a different reference count from the "proper" one, and one of them will end up losing and holding a dangling reference when the object is deleted.
enable_shared_from_this 已成为 C++ 11 标准的一部分.您也可以从那里和 boost 获取它.
enable_shared_from_this has become part of C++ 11 standard. You can also get it from there as well as from boost.

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