Re: Factoring SFINAE tests

From:
 xtrigger303@gmail.com
Newsgroups:
comp.lang.c++
Date:
Mon, 10 Sep 2007 14:23:42 -0000
Message-ID:
<1189434222.908773.129960@g4g2000hsf.googlegroups.com>
On Sep 10, 4:06 pm, Barry <dhb2000@gmail.com> wrote:

xtrigger303@gmail.com wrote:

On Sep 8, 2:17 pm, Greg Herlihy <gre...@pacbell.net> wrote:

On 9/7/07 3:42 PM, in article
46e1d3f5$0$28521$5a62a...@per-qv1-newsreader-01.iinet.net.au, "Gianni

Mariani" <gi3nos...@mariani.ws> wrote:

Below is an attempt to factorize the "SFINAE" idiom. I have tried it on
three compilers and they all complain. As far as I can tell, it's valid
code. The questions are, are the compilers right to reject the code or
is the code right and why ? and has anyone successfully mastered the act
of factorizing SFINAE?
----------------------------------------------------------------------
template <template <typename> class w_Test, typename w_Type>
class sfinae_test
{
     typedef char (&false_value)[1];
     typedef char (&true_value)[2];
     struct base {};
     struct derived : base {};
     static derived & make_derived();
     static w_Type& make_test();
     template <bool i>
     struct H
     {
     };
     template <typename Z>
     static false_value func( Z &, base & );
     template <typename Z>
     static true_value func(Z&, derived&, typename w_Test<Z>::type* =0 );
public:
     static const bool value =
         sizeof(func(make_test(), make_derived())) == sizeof(true_value);
};
// some tests
// is_defined_test will fail if T is not defined
template <typename T> class is_defined_test
{
     typedef char (&type)[ sizeof( T ) ];
};
// has_member_orange will fail if T had no orange
template <typename T> class has_member_orange
{
     static T& make_T();
     public:
     typedef char (&type)[ sizeof( make_T().orange ) ];
};

There is no example of SFINAE ("substitution failure is not an error") in
the this program; both has_member_orange<A> and is_defined_test<B> match
func()'s second parameter:

    typename w_Test<Z>::type

because both template classes do declare an interior typedef named "type".
Now, the compiler does run into trouble instantiating has_member_orange<A>
and is_defined_test<B> (because the "type" typedef is ill-formed)- but this
error occurs only after the type deduction needed to select func() has
already been performed. So the ill-formed template specialization error
comes too late for SFINAE to help - so the result is a compile-time error.

Greg


Hi to all,
sorry for bringing this out again but it was just too interesting.
I did some research to understand what you guys were talking about and
I got to this link

http://aszt.inf.elte.hu/~gsd/slides/Introspection.ppt


Good

I think it is a presentation of a book...
It's quite cryptic but I used it as a starting point. Hope it's not
plagiarizing... :-)


abslooootely, who are you gonna pay?
:-)

Obviously I dind't get anywhere interesting because I miserably failed
to "factorize" these idioms with templates, inheritance, etc...
But I've done some things that I did not know were possible with
macros ( probably I just reinvented the wheel )...
Please let me know what you think and if there is a better way of
doing it. I've just done some small trivial testing...


which compiler you testes?

I copy and paste, it's quite a mess(as my reader wraps the line a lot),
and after a long time reformatting, as there are toooo many macros, I
can't get it work with Comeau online. Or still I miss something on the
macro! err...

--
Thanks
Barry


Ooops, too many evil macros... I knew it.. ;-)
if anyone is vaguely interested in giving me advices on this I can
send the file...
Let me know
Francesco

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