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

From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2008-07-27 14:21:59

Steven Watanabe wrote:
> AMDG
> 
> Mathias Gaunard wrote:

>> If they're not real rvalues, that means casting was used. Which should
>> then make it explicit that trying to access it later on is undefined
>> behaviour as far as the implementation of the type is concerned.
>>   
> 
> Such a move would be much too limiting.

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?

Were some guidelines chosen by the standard committee as to what move
semantics ought to do?

> Consider how std::remove would be implemented (untested):
> 
> template<class Iter, class T>
> Iter remove(Iter begin, Iter end, const T& value) {
>    begin = std::find(begin, end, value);
>    Iter write_pos = begin++;
>    for(; begin != end; ++begin) {
>        if(*begin != value) {
>            *write_pos++ = std::move(*begin);

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.

>        }
>    }
>    return(write_pos);
> }

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