Re: [Boost-users] Re: shared_ptr, intrusive_ptr ...

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-05-15 14:46:56

Jean-François Brouillet wrote:

> The following is a perfectly legal Java idiom which I
> find all over the place in the source I am porting.
>
> Basically it says: For simple use, just use me as is.
> I implement a default policy regarding so and so. However
> you are free to specify your own policy when required:
>
>
>     interface I {
>         void i() ;
>     }
>
>     class C implements I {
>
>         I myI ;
>
>         void i() {
>             System.out.println("I.i() called, as implemented by C") ;
>         }
>
>         C() {
>             myI = this ;
>         }
>
>         void
>         setI(I i) {
>             myI = i ;
>         }
>
>         void f() {
>             myI.i() ;
>         }
>     }

You can do this with shared_ptr, but not with intrusive_ptr. ;-)

class C: private I
{
    shared_ptr<I> myI;

    void i(); // ...

    C( C const & );
    C& operator=( C const & );

public:

    C(): myI( this, null_deleter() )
    {
    }

    void setI( shared_ptr<I> const & i )
    {
        myI = i;
    }
};

A null_deleter works here because you know that the internal I and the 
shared_ptr member have the same lifetime, that of C.

http://boost.org/libs/smart_ptr/sp_techniques.html#in_constructor

"Depending on context, if the inner shared_ptr this_ doesn't need to keep 
the object alive, use a null_deleter as explained  here and here."

http://boost.org/libs/smart_ptr/sp_techniques.html#static