什么是enable_shared_from_this的用处 [英] what is the usefulness of enable_shared_from_this

问题描述

我碰到 enable_shared_from_this 跑一边读Boost.Asio的例子和阅读，我还是失去了这个应该怎么正确使用的文档后。是否有人可以给我使用这个类时的一个例子和/或与解释是有道理的。

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/or and explanation of when using this class makes sense.

推荐答案

它使您能够得到一个有效的的shared_ptr 实例这个，当你已经是这个。没有它，你就没有得到一个的shared_ptr 到这个，除非你已经有一个作为成员的方式。从enable_shared_from_this的 Boost文档这个例子的：

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
}

该方法f（）的返回一个有效的的shared_ptr ，即使它没有成员实例。需要注意的是，你不能简单地做到这一点：

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);
    }
}

共享指针，这回将有来自正确的一个不同的引用计数，和其中一人将最终失去并保持当对象被删除悬空引用。

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 将是新的C ++ 0x标准的一部分一样，所以你也可以从那里，以及从升压得到它。

enable_shared_from_this is going to be a part of the new C++0x standard as well, so you can also get it from there as well as from boost.

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