Re: [boost] boost::mpl::for_each and value_initialized

Subject: Re: [boost] boost::mpl::for_each and value_initialized
From: Niels Dekker - address until 2010-10-10 (niels_address_until_2010-10-10_at_[hidden])
Date: 2010-01-21 13:20:59

>> Do I understand correctly that you want the memset call to be
>> skipped for those compiler versions that have implemented
>> value-initialization entirely correctly?

Peter Foelsche wrote:
> I'm only concerned with boost::mpl::for_each
> This function calls a function object by passing an object of the
> matching type.
> The contents of this object usually do not matter -- and in case they
> do, the programmer can write a default constructor to initialize this
> object. So there is no need in boost::mpl::for_each to call
> value_initialized.

So you think the issue can be resolved properly, merely by modifying 
mpl/for_each.hpp, right? You proposed at 
http://lists.boost.org/Archives/boost/2009/09/155805.php to modify 
mpl/for_each.hpp by changing:

! value_initialized<arg> x;
! aux::unwrap(f, 0)(boost::get(x));

Into:

! aux::unwrap(f, 0)(arg());

I see some subtle differences here. First of all, boost::get(x) return a 
reference to an lvalue, while arg() is an rvalue. Do you think that's okay?

Secondly, things *might* go wrong if arg doesn't have a user-declared 
default-constructor *and* the compiler hasn't correctly implemented 
value-initialization. That's why Fernando and I added the memset call to 
boost::value_initialized, of course. But you think the data members of the 
temporary arg() won't be accessed anyway, right?  Unless arg has a 
user-declared default-constructor?

I'm sorry I don't know mpl/for_each.hpp well enough.  But if the answer to 
each of the above questions is yes, I guess your proposal is fine.

  Kind regards, Niels

--
Niels Dekker
http://www.xs4all.nl/~nd/dekkerware
Scientific programmer at LKEB, Leiden University Medical Center