Re: [boost] C++0x question

From: Marco Costalba (mcostalba_at_[hidden])
Date: 2007-10-21 00:29:50

On 10/18/07, Doug Gregor <dgregor_at_[hidden]> wrote:
>
> On Oct 16, 2007, at 5:58 AM, Marco Costalba wrote:
>
> > Starting to hack with boost libraries I found myself more and more
> > stumbling across a big brick of C++ language, i.e. SFINAE works on
> > types only, not on expressions.
> >
> > If an expression is ill formed there's no SFINAE around that will
> > avoid you a barfing compiler.
>
> This issue has been actively discussed in the C++ committee. You can
> track it's progress via core issue 339 on the Core Issues list, which
> is here:
>
>         http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#339
>
> At the very bottom of the issue text, there is a note that describes
> the latest view on the matter. The resolution we're likely to get is
> that SFINAE will be extended to support expressions. There are still
> cases that will cause "hard" failures (not SFINAE failures), for
> example if one ends up trying to instantiate a class template inside
> a sizeof() expression and that instantiation fails.
>

>From above cited document:

"We decided it's easier to write the definition by listing the errors
that are not treated as SFINAE failures, and the list we came up with
is as follows:

1-errors that occur while processing some entity external to the
expression, e.g., an instantiation of a template or the generation of
the definition of an implicitly-declared copy constructor"

This limitation seems quite important. As example in the code I posted
in this thread:

template<int n, typename arg>
void dispatch_on_value(std::string const& s, arg const& a)
{
   if ( s == some_string(n) )

       /* calls member foo() on the class returned  by get_a_class */
       return get_a_class<n>().foo(a);

  return dispatch_on_value<n-1>(s, a); // try with next one
}

I could think to use sizeof as a specialization somewhere to filter
out invalid functions, something like:

sizeof(get_a_class<n>().foo(arg()))

But, if I have read correctly the proposed change, this would result
in being still considered an error, not under SFINAE case.

IMHO closing the doors to instantiation of a class template inside a
sizeof() expression could cut away a good number of possible cases,
also not easy to foressen ones (i.e. from where language 'progress'
normally comes from).

In my case, if you have a compile time set of functions/classes and
you want to choose at run-time one among them using a run-time entity
(variable, arguments values, etc..) a possible way would be to create
at compile time a chain of all the instantations of your set, linked
with recursion, then at run-time return from the recursion chain at
the point your run-time requirements are met.

But this works only if at compile-time it would be possible to discard
the ill-formed ones when instantiating the recursive chain.

Am I missing something?

Thanks
Marco