Re: [boost] [shared_ptr] dangerous implementation of moveconstructor

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-04-16 15:06:53

Ion Gaztañaga wrote:

> For objects that have no such default-state, and have no resource
> reassignment functions (like reset() or similar) there is no problem
> because there is no interface to reuse the object.

I think I was with you until this point. It seems to me that the exact 
opposite is true; objects that have no "empty" or "default" state cannot 
implement a move, at least not easily, whereas reusing a moved-from object 
via assignment or reset is no problem at all.

FWIW, I've changed the shared_ptr move constructors and assignment operators 
to leave the source empty, but this doesn't mean that I'm convinced that the 
general case needs to be constrained in such a way. In particular,

    T& operator=( T && r )
    {
        swap( r );
        return *this;
    }

seems a very reasonable implementation of move assignment, and has a slight 
performance advantage (one call to ~T instead of two in a typical use case) 
over

    T& operator=( T && r )
    {
        T( move( r ) ).swap( *this );
        return *this;
    }

which is what is needed for r to be left empty.