Re: [boost] [proposal] raw move

Subject: Re: [boost] [proposal] raw move
From: Eric Niebler (eric_at_[hidden])
Date: 2011-08-23 10:42:53

On 8/23/2011 8:06 AM, Julian Gonggrijp wrote:
<snip>
> PART I.
> I think the default templates header should look like this:
> 
> template <class T>
> struct underlying_type_traits {
>     typedef T type;
> };

Only if T is a POD, so BOOST_MPL_ASSERT((is_pod<T>)) would be a useful
addition here.

> #define UNDERLYING_TYPE(T) underlying_type_traits< T >::type
> 
> template <class T>
> void move_raw (T& source, T& target) {
>     target = source;
> }
> 
> template <class T>
> void move_raw (T& source, typename UNDERLYING_TYPE(T)& target) {
>     target = source;
> }
> 
> 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.

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.

> // Not shown: partial specializations for arrays, like in Boost.Swap.
> // Possible refinement: partial specializations that disable 
> // compilation for const types.
> 
> There are many good reasons to default to type identity and copy 
> assignment:
> 
> 1. it is optimal for PODs;
> 2. it works and is safe for copyable non-PODs;

No, it's not safe.

> 3. from 1. and 2.: the algorithms from part II. are automatically 
>     guaranteed to work for all types that can currently use the copy-
>     based counterparts from the STL;
> 4. on top of 3., all algorithms from part II. are guaranteed to be 
>     always at least as efficient as their copy-based counterparts;
> 5. from 3. and 4.: raw_move::swap can directly replace std::swap as 

What's raw_move::swap?

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com