Re: [boost] formal review request : mcs::units

From: Matthias Schabel (boost_at_[hidden])
Date: 2007-02-10 13:35:05

Hi Andrey,

> Maybe something like this?
>
> // the_library.hpp
>
> template< unsigned int N = 0 >
> struct ordinal :
>   public mpl::int_< N >
> {
>   typedef ordinal< N + 1 > next;
> };
>
> // We may use ordinals in the lib, in that case
> // initial_ordinal below should be the first
> // available ordinal for users
>
> typedef ordinal< > initial_ordinal;
>
>
> // user_code.hpp
>
> struct tag1 : public initial_ordinal {};
> struct tag2 : public tag1::next {};
> struct tag3 : public tag2::next {};

The problem with this is that you still need to keep track of previously
defined tags. That is, you need to know what the last tag defined was to
call ::next on it. Imagine that you wanted to extend the SI system  
with a
new fundamental unit for fabulousness. As it stands now, you would do
something like this in your code:

namespace boost {
namespace units {

struct fabulousness_tag : public ordinal<10> { };

typedef fundamental_dimension<fabulousness_tag>::type	fabulousness_type;

namespace SI {

// SI unit of fabulousness
typedef unit<SI::system,fabulousness_type>		fabulousness;

static const fabulousness	fab,
                                                fabs;

} // namespace SI
} // namespace units	
} // namespace boost

Here, you need to know that there are already 9 tags defining the pre-
existing fundamental types in the SI system. What I want to do is to
query the compiler as to how many previous distinct instantiations of
ordinal<N> have already been encountered so you could write

struct fabulousness_tag : public ordinal<N_prev_ordinal_instantiations 
+1> { };

This would allow you to add tags without ever having to look around
to determine the current upper limit...If may not be possible, though.

Matthias