Re: [boost] gcc4.3 warning for multiple scope exits

Subject: Re: [boost] gcc4.3 warning for multiple scope exits
From: Alexander Nasonov (alnsn_at_[hidden])
Date: 2009-07-14 09:38:56

Kim Barrett <kab.conundrums <at> verizon.net> writes:

> 
> At 10:48 AM +0000 7/14/09, Alexander Nasonov wrote:
> >boost::scope_exit::aux::holder<
> >   boost::scope_exit::aux::declare<
> >     sizeof(boost_scope_exit_args)
> >   >::apply<0>::apply_value
> >>  boost_scope_exit_args;
> >
> >If sizeof(boost_scope_exit_args) is equal to sizeof(declared),
> >the resulting type would be holder<(0<0>::apply_value)>. But I don't
> >see a global variable apply_value in your patch.
> 
> The added parenthesis in the above, in
> 
>    holder<(0<0>::apply_value)>
> 
> don't describe the actual parse and evaluation. Instead, that snippet
> gets parsed and evaluated as
> 
>    holder<(0<0)>::apply_value
> 

This is crazy, isn't it?

Q: correctly indent the following code: <see code above>
A: it's impossible

For the second use, the correct indentation is:

boost::scope_exit::aux::holder<
  boost::scope_exit::aux::declare<
    sizeof(boost_scope_exit_args)
  >::apply < 0
>::apply_value > boost_scope_exit_args;

> >BTW, I modified Steven's code and introduced a bug because
> >sizeof(declare<sizeof(undeclared)>::apply<0>) does not necessarily
> >equal to sizeof(declared).
> 
> Are you sure? Perhaps I'm missing something, but it looks to me like
> those values should indeed be the same. The "apply" class template in
> question has one member, of type "declared". Given that, I would be
> surprised if the "apply" class template could have a different size
> (or alignment) from the "declared" type.

They're the same because void* is well aligned and there is no padding
in struct apply<0> but strictly speaking, they don't have to be of equal
size. I like explicit specializations for declared and undeclared, though.
They make reading code a little bit easier (and here "a little bit" make
a big difference, given how complex the trick is).

Perhaps adding BOOST_STATIC_ASSERT would be enough?

Alex
multiplied by a complec.