enable_shared_from_this（c ++ 0x）：我做错了什么？ [英] enable_shared_from_this (c++0x): what am I doing wrong?

问题描述

我只是在即将到来的新c ++标准的智能指针。但是我无法掌握shared_from_this函数的用法。这里是我有：

I'm just toying around with the smart pointers in the upcoming new c++ standard. However I fail to grasp the usage of the shared_from_this function. Here is what I have:

#include <iostream>
#include <memory>

class CVerboseBornAndDie2 : public std::enable_shared_from_this<CVerboseBornAndDie2>
{
public:
    std::string m_Name;
    CVerboseBornAndDie2(std::string name) : m_Name(name)
    {
        std::cout << m_Name << " (" <<  this << ") is born!" << std::endl;
    }
    virtual ~CVerboseBornAndDie2()
    {
        std::cout << m_Name << " (" <<  this << ") is dying!" << std::endl;
    }
};

int main(){
    CVerboseBornAndDie2* vbad = new CVerboseBornAndDie2("foo");
    std::shared_ptr<CVerboseBornAndDie2> p = vbad->shared_from_this();
}

并且它在行中引发std :: bad_weak_ptr异常

and it throws a std::bad_weak_ptr exception in the line

std::shared_ptr<CVerboseBornAndDie2> p = vbad->shared_from_this();

如果我改为

std::shared_ptr<CVerboseBornAndDie2> p(vbad);

它可以工作，然后可以

std::shared_ptr<CVerboseBornAndDie2> p2 = p.get()->shared_from_this();

因此，在使用shared_from_this之前，对象必须属于一个shared_ptr吗？

so must the object belong to one shared_ptr before I can use shared_from_this? But how can I know this beforehand?

推荐答案

这是使用 shared_from_this 必须存在至少一个拥有相关对象的 shared_ptr 。这意味着您只能使用 shared_from_this 来检索拥有引用或指针的对象的 shared_ptr ，你不能使用它来确定这样的对象是否由 shared_ptr 拥有。

It is a precondition of using shared_from_this that there must exist at least one shared_ptr which owns the object in question. This means that you can only use shared_from_this to retrieve a shared_ptr that owns an object to which you have a reference or pointer, you cannot use it to find out if such an object is owned by a shared_ptr.

您需要返工你的设计，使你或者你保证任何这样的对象是由一个 shared_ptr 管理，或者你不需要知道或最终（最小希望）你创建一些其他管理这种知识的方式。

You need to rework your design so that either you are guaranteed that any such object is being managed by a shared_ptr or that you don't ever need to know or finally (and least desirably) you create some other way of managing this knowledge.

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