Re: [Boost-users] [smart_ptr] Resetting an intrusive_ptr

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-05-02 07:29:59

Scott Meyers wrote:
> Suppose I have an auto_ptr, shared_ptr, or scoped_ptr.  If I want to
> set it to null, I do this (assuming the pointer is named p):
>
>   p.reset();          // okay
>
> If I try this,
>
>   p = 0;              // error
>
> I get an error during compilation.  In other words, reset works, but
> assignment of the null pointer does not.
>
> But suppose p is an intrusive_ptr.  Now
>
>   p.reset();          // error
>
> fails, and
>
>   p = 0;              // okay
>
> succeeds.  In other words, assignment works, but reset does not.  I
> find this, er, unintuitive.  Does anybody know why intrusive_ptr
> marches to its own nullifying drummer?

The reason for p = 0 working in intrusive_ptr's case is that intrusive_ptr 
has an assignment operator taking a raw pointer. The other smart pointer 
types do not.

The lack of intrusive_ptr::reset is arguably a defect in intrusive_ptr's 
interface. My personal opinion is that moving between shared_ptr and 
intrusive_ptr doesn't have to be smooth because their semantics are 
different enough to warrant a careful code review after the switch. On the 
other hand, consistency does have its benefits.