Re: [boost] Possible Placeholder problem with transform & if_ (?)

From: Brian Braatz (brianb_at_[hidden])
Date: 2005-07-27 18:04:27

Ok - I just fixed this- I don't understand WHY it fixed it but it did.

I moved the if_<> into a metafunction class and passed the metafunction
class to transform.

I would think that either using it inline or as a mfc would be
equivalent.

(out of curiosity)- Anyone know the answer to this why this is the case?

> -----Original Message-----
> From: Brian Braatz
> Sent: Wednesday, July 27, 2005 3:41 PM
> To: 'boost_at_[hidden]'
> Subject: MPL:Possible Placeholder problem with transform & if_ (?)
> 
> Either something strange is happening with arg<> and transform or I
did
> not write this code correctly. ( :) )
> 
> Any help pointing me to either conclusion appreciated. The next step
is to
> try the same algorithm with transform_view. I really don't want a lazy
> sequence (how could mother be proud?), but sometimes life is
> disappointing.
> 
> Thanks Guys.
> 
> CODE:
> 
> struct print_type
> {
> 	std::ostream & m_os;
> 	print_type(std::ostream & os = std::cout)
> 	: m_os(os)	{}
> 
>     template <typename T>
>     void operator()(T const& v) const
>     {
>         m_os << "[ " << typeid(v).name() << " ] ";
>     }
> };
> 
> template <class seq_t>
> void _print_types(const char * pHeading)
> {
> 	std::cout << "** " << pHeading << std::endl;
> 	mpl::for_each<seq_t>(print_type());
> 	std::cout << std::endl;
> }
> #define print_types(seq) _print_types<seq>(#sew);
> 
> template <class T>
> struct YES
> {
> 	typedef YES type;
> };
> template <class T>
> struct NO
> {
> 	typedef NO type;
> };
> 
> struct A{};
> 
> typedef mpl::vector<A> vec_t;
> 
> typedef mpl::transform
> <
> 	vec_t,
> 	mpl::if_
> 	<
> 		boost::is_same<mpl::_1,	A >,
> 		YES< mpl::_1  >::type,
> 		NO< mpl::_1  >::type
> 	>::type
> >::type result;
> 
> void test()
> {
> 	print_types (result);
> 
> 	/* //////////////////////////
> 
> 	PRINTS:
> 
> 	** result
> 	[ struct NO<struct A> ]
> 
> 	SHOULD PRINT:
> 
> 	** result
> 	[ struct YES<struct A> ]
> 
> 	////////////////////////////
> 	*/
> }