enable_shared_from_this (c++0x): what am I doing w

Question

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();
}

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

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

if I instead do

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

it works and I can afterwards do

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

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

Answer 1

To extend Charles answer, when you use enable_shared_from_this you usually want something like below in order to guarantee that there exists a shared_ptr.

class my_class : public std::enable_shared_from_this<my_class>
{
public:
    static std::shared_ptr<my_class> create() // can only be created as shared_ptr
    {
         return std::shared_ptr<my_class>(new my_class());
    }
private
    my_class(){} // don't allow non shared_ptr instances.
};

Answer 2

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.

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.