Re: How safer to use boos::shared_from_this() from derived class inherited from based class?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2019-08-02 06:45:55

JH wrote:
> Hi,
>
> I have a derived base class:
>
> class Base : boost::enable_shared_from_this<Base> {
> public:
>     Base();
>     virtual ~Base();
>     virtual boost::shared_ptr<Base> SharedFromThis(void) {return 
> shared_from_this();}
> private:
> }
>
> class Derived : public Base {
> public:
>     Derived();
>     ~Derived();
>
> //    boost::shared_ptr<Derived> SharedFromThis(void) {return 
> boost::static_pointer_cast<Derived>(shared_from_this());}
> private:
>
> };
>
> Applications are sharing the derived class, it can either use 
> shared_ptr<Derived> or shared_ptr<Base> by calling the SharedFromThis(), 
> three options:
>
> (1) Don't define derived class own SharedFromThis(), always call Base 
> SharedFromThis() to pass shared_ptr<Base> as Derived shared_ptr, the 
> reference count will be shared_ptr<Base>.
>
> (2) Define derived class its own SharedFromThis() and pass 
> shared_ptr<Derived> to applications:
>
> boost::shared_ptr<Derived> SharedFromThis(void) {return 
> boost::static_pointer_cast<Derived>(shared_from_this());}
>
> That one cannot be virtual as conflicted to Base SharedFromThis(), the 
> reference count will always be boost::shared_ptr<Derived>
>
> (3) Mixed both boost::shared_ptr<Derived> and boost::shared_ptr<Base> in 
> applications, that could be a bad option to cause segmentation as one 
> shared_ptr could be ended before another shared_ptr.
>
> Which option will be safer?

The shared_ptr<Derived> that is returned by the static_pointer_cast shares 
the same reference count as the original shared_ptr<Base>, so it shouldn't 
matter.