Re: [boost] [range] How to extend Boost.Range?

From: Eric Niebler (eric_at_[hidden])
Date: 2005-09-09 12:24:07

Thorsten Ottosen wrote:
> 
> Btw, my range_size uses this code
> 
> 		template< class T >
> 		struct add_unsigned;
> 
> 		template<>
> 		struct add_unsigned<short>
> 		{
> 			typedef unsigned short type;
> 		};
>     
> 		template<>
> 		struct add_unsigned<int>
> 		{
> 			typedef unsigned int type;
> 		};
> 
> 		template<>
> 		struct add_unsigned<long>
> 		{
> 			typedef unsigned long type;
> 		};
> 
> #ifdef BOOST_HAS_LONG_LONG
> 		
> 		template<>
> 		struct add_unsigned<long long>
> 		{
> 			typedef unsigned long long type;
> 		};
> #endif
> 
> Can anybody spot weaknesses in that approach?
>  

Well, you haven't defined a primary template, so add_unsigned<unsigned> 
will not compile. IMO, your primary template should "typedef T type;". 
But depending on what you're trying to do, this may work, and it's a lot 
shorter:

template< typename Int >
struct add_unsigned {
     typedef typename boost::uint_t<
         sizeof(Int) * CHAR_BIT
     >::least type;
};

> 
>>Regarding the concepts, it is correct to say, as you do, that the calls 
>>to begin(), end(), et al., must be qualified by boost::. But they also 
>>must say that the following
> 
> 
> <quote> 
> 
>>is also well formed and has the same meaning:
>>
>>     using namespace unspecified-namespace;
>>     boost_range_begin(x);
> 
> </quote>
>  
> 
>>I think you should also say somewhere (but not necessarily in the 
>>Concepts section) that unspecified-namespace contains the 
>>implementations of boost_range_begin(), et al., for the std containers, 
>>std::pair, arrays and null-terminated strings. I think that should do 
>>it. Does anybody have a better suggestion?
> 
> 
> The Range concepts have been a fruitful source of confusion. I'm still myself
> a bit puzzled sometimes :-)
> 
> We want to say a range r supports
> the expressions
> 
> boost::begin(r)
> boost::end(r)
> boost::size(r)
> 
> That is true fo certain types if we do include a certain header, otherwise it 
> is not. That has really irritated me: in one translation-unit T could be
> conforming to a range, in others it need not to.

If you want to treat a type as a range, you need to include the file 
that makes that type a conforming range. I don't see any problem with that.

> 
> If we use your quote above, it seems to me that the to expressions
> are not equivalent: you can't exchange one with the other.

Why not?

> 
> A Range concept is composed of more than the concept defining
> boost_range_begin()/boost_range_end().

Of course. And I'm saying that for a type to conform to the range 
concept, it must do more than ensure that boost::begin(), boost::end(), 
and boost::size() are well-formed and have the proper semantics. I'm 
suggesting that you *add* the following requirements _in addition to_ 
the ones already listed:

using namespace unspecified-namespace;
boost_range_begin(r);

is well-formed and has the same semantics as boost::begin(r). And the 
same for boost_range_end() and boost_range_size().

> 
> We might want to say the following:
> 
> T models a Range if it models one of the following concepts
> 
> 

???

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com