Re: [boost] [proposal] raw move

Subject: Re: [boost] [proposal] raw move
From: Julian Gonggrijp (j.gonggrijp_at_[hidden])
Date: 2011-08-23 11:35:17

Eric Niebler wrote:

> On 8/23/2011 8:06 AM, Julian Gonggrijp wrote:
> <snip>
>> [...]
>> template <class T>
>> void move_raw (typename UNDERLYING_TYPE(T)& source, T& target) {
>>    target = source;
>> }
> 
> Yikes, no. T's copy constructor can throw. This should be a move, and
> then only if T's move assign cannot throw. Otherwise, move_raw should
> probably be undefined.

If move_raw is implemented as a copy, it doesn't really matter if the 
copy throws because the source object is left in a valid state, right? 
I mean, with the copy-based swap we usually also don't worry about the 
possibility that one of the copies might throw...

> You may decide to provide an easy way for users to opt-in for a memcpy
> based solution, but that option should only be available only on
> compilers for which memcpy-ing non-PODs is empirically known to work.
> And there should be a warning about the non-portability about doing that.

If you are right and using copy as a fallback for raw moves is not an 
option, it means a significant loss of generality. Adding an opt-in 
for a memcpy hack will give only a very small relief. But perhaps such 
a less-general tool can still have its use.

> What's raw_move::swap?

It was an improvised shorthand for 'the swap algorithm that is 
implemented with raw moves'.

-Julian