[boost] Re: [type_traits] fully qualified names

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2005-03-10 17:56:40

Jody Hagins wrote:
> The coding guidelines document (boost/more/int_const_guidelines.htm),
> states the following...
> 
> 
> Always use a fully qualified name to refer to an integral constant
> expression.
> 
> For example:
> 
> typedef myclass< ::boost::is_integral<some_type>::value> mytypedef;
> 
> Rationale: at least one compiler (Borland's), doesn't recognise the name
> of a constant as an integral constant expression unless the name is
> fully qualified (which is to say it starts with ::).
> 
> 
> 
> However, it appears that the same compiler (BCB6) has problems if the
> '::' is left in.  For example...
> 
> 
> template <class T>
> struct Foo
> {
>   typedef typename ::boost::remove_reference< T >::type type;
>   typedef typename ::boost::add_reference<
>       typename ::boost::add_const< type >::type
>       >::type ref_to_const_type;
> };
> 
> fails to compile.  Initially I thought it had something to do with
> typename (e.g., another BOOST_DEDUCED_TYPENAME) because, the first,
> simpler line compiles if I remove the "typename" keyword. 
> Unfortunately, the following does not compile either
>

Well, this is neither a deduced context nor does it involve ICEs.

Besides, as you might know, BCB has problem detecting dependent types 
and thus removing typename is always a good idea before suspecting other 
evil.

There is really no point in always fully qualifying every name you use.
Only do so when referring to a static member constant!

In fact, the easiest way is to avoid non-type template arguments 
whenever possible and pass around types instead (see MPL Integral 
Constant Concept).

When the compiler substitutes a template type paramter with its argument 
it is "fully qualified automatically":

   template<typename IC> struct X
   {
     // using IC::value is fine, here
   }

This technique could make some Type Traits code more readable, I guess. 
Further, some passages might be overly careful..

> 
> So, I am left scratching my head, wondering what to do if I want to use
> TypeTraits in a portbale manner.
> 

Fully qualify only when referring to static member constants.

Regards,

Tobias