`enable_shared_from_this`的作用是什么? [英] What is the usefulness of `enable_shared_from_this`?

问题描述

在阅读Boost.Asio示例时遇到了enable_shared_from_this，在阅读了文档之后，我仍然迷失了如何正确使用它的方法.有人可以给我一个例子，解释使用此类的合理性.

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

当您拥有的全部是this时，它使您可以将有效的shared_ptr实例获取到this.没有它，您将无法将shared_ptr升级为this，除非您已经有一个成员.来自关于enable_shared_from_this 的增强文档的示例:

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()返回有效的shared_ptr，即使它没有成员实例.请注意，您不能简单地这样做:

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);
    }
}

此共享指针返回的共享指针的引用计数与正确"的共享指针的引用计数不同，并且其中的一个指针最终将丢失并在删除对象时保持悬挂的引用.

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.您也可以从那里获得它，也可以从中获得它.

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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