Re: [boost] [interest] underlying type library

Subject: Re: [boost] [interest] underlying type library
From: Julian Gonggrijp (j.gonggrijp_at_[hidden])
Date: 2011-08-20 13:33:48

Mathias Gaunard wrote:

> On 08/20/2011 06:09 PM, Julian Gonggrijp wrote:
> 
>> UNDERLYING_TYPE(T) represents a type which is (conceptually speaking)
>> a POD-type with the same member data (and hence the same alignment) as
>> T.
> 
> You mean like boost::aligned_storage<sizeof(T), boost::alignment_of<T>::value> ?

Pretty much. I think it could do the job.
(Although of course it's not THE underlying type in a very strict 
conceptual sense.)

>> template<class T>
>> void swap (T&  x, T&  y) {
>>     UNDERLYING_TYPE(T) temp;
>>     move_raw (x, temp);
>>     move_raw (y, x);
>>     move_raw (temp, y);
>> }
> 
> This clearly looks like undefined behaviour.

It isn't. :-)
The code is guaranteed to swap x and y and leave them in a valid 
state. temp will be discarded like any POD object, without any chance 
for unwanted side effects.
For clarity, I have already implemented and tested this and everything 
works as expected.

> You cannot use bitwise copy to implement move semantics.
> That just doesn't work.

It probably depends on what you mean by 'move semantics' and what you 
mean by 'implement'. What I'm sure of, is that my library can 
(unintrusively) simulate move constructors, move destructors and the 
returning of rvalue references from functions. Of course, it doesn't 
use *only* bitwise copies for that purpose and the type involved needs 
to be default-constructible.

Why do you think it just cannot work?

Note that I'm not claiming to deliver a complete library for move 
semantics -- to the contrary. The underlying type library doesn't 
predate on Boost.Move; I only think the underlying type library could 
be useful in the implementation of Boost.Move.

For a more thorough discussion please refer to the lecture notes of 
Alexander Stepanov. He explains these things much better than I could
do. :-)

-Julian

http://www.stepanovpapers.com/notes.pdf