Re: [boost] [unnullable_ptr] Proposal for a new library

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-07-28 14:42:45

AMDG

Mathias Gaunard wrote:
> Your code needs to be able to assign a non-empty object to an empty one.
> Would you also consider it necessary to be able to assign an empty
> object to a non-empty one?
>   

To be more specific my code needs to assign a non-empty object
to one that may or may not be empty.

> Possible alternative here: destruct and construct.
>
> replace
> *write_pos++ = std::move(*begin);
> by
> reassign(*write_pos++, std::move(*begin));
>
> with
>
> template<typename T1, typename T2>
> void reassign(T1& old, T2&& new)
> {
>     old.~T1();
>     new(&old) T1(std::forward<T2>(new));
> }
>
> A function called `reassign' (that would destroy the old object then
> construct the new one) could be used instead of assignment in cases
> where the left operand can be in a `moved' state.
>   

This only provides yet another way to assign (And to get undefined
behavior if you forget to use the right function).

a = b; // a must not have been moved from
a = move(b); // ditto
reassign(a, b); // a may have been moved from.  doesn't even provide 
basic exception safety.
reassign(a, std::move(b)); // a may have been moved from.  Exception 
safe as long as the move constructor can't throw.

I thought that you were advocating safety?

> Anyway, I'm pretty sure that function could be purely implemented in
> terms of swap, which seems like a safer construct.
>   

Sure it could be implemented in terms of swap, but I disagree that
swap is safer than proper move assignment.  swap is also less efficient
for POD.

In Christ,
Steven Watanabe