[boost] Re: scoped_ptr::reset and auto_ptr

From: Ed Brey (brey_at_[hidden])
Date: 2002-09-30 10:32:40

"Peter Dimov" <pdimov_at_[hidden]> wrote in message news:014001c26890$f82f8b80$1d00a8c0_at_pdimov2...
> From: "Ed Brey" <brey_at_[hidden]>
> > The constructor for boost::scoped_ptr's has an overload for std::auto_ptr, which is good,
> > IMHO.  However, its reset member has no similar overload.  This seems like an oversight.
> > Is it?
> 
> The motivating case for this constructor was
> 
> std::auto_ptr<X> createX();
> 
> scoped_ptr<X> px(createX());
> 
> Nobody requested a reset(auto_ptr) method. FWIW, shared_ptr uses operator=
> for this purpose, and not reset(), but scoped_ptr has no assignment
> operators.

Here is the equivalent motivating example for the reset overload:

struct A {
    scoped_ptr<X> px_;

    initialize(std::auto_ptr<X>& p) {
        px_.reset(p);
    }
};

The argument for why the member variable should be a scoped_ptr (versus an auto_ptr) is the same as the argument for why the automatic variable is a scoped_ptr in your example.

The thought of an operator= on scoped_ptr that takes an auto_ptr is an interesting one.  Given that its constructor takes an auto_ptr, one could say that scoped_ptr will "transfer in a pointer", but will not "transfer out a pointer".  Given that definition of scope, an assignment operator that transfers in a pointer seems necessary for completeness.