RE: [boost] Re: FW: converting shared_ptr<const T> to shared_ptr< T>

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2001-03-04 23:51:43

David Abrahams wrote:
> No, because remove_cv<> doesn't work without partial 
> specialization. 

Strictly speaking, that's true; I just wanted to note that the latest
version of the 'type traits' library (in CVS) makes the above assertion not
so flat. From the library documentation:

...support for partial specialization of class templates is required to
correctly implement the type transformation templates. On the other hand,
practice shows that many of the templates from this category are very
useful, and often essential for implementing some generic libraries. Lack of
these templates is often one of the major limiting factors in porting those
libraries to compilers that do not yet support this language feature. As
some of these compilers are going to be around for a while, and at least one
of them is very wide-spread, it was decided that the library should provide
workarounds where possible. The basic idea behind the workaround is

1. To manually define full specializations of all type transformation
templates for all fundamental types, and all their 1st and 2nd rank
cv-[un]qualified derivative pointer types, and to 
2. Provide a user-level macro that will define such explicit specializations
for any user-defined type T. 

The first part guarantees the successful compilation of something like this:

BOOST_STATIC_ASSERT((is_same<char, remove_reference<char&>::type>::value));
...
BOOST_STATIC_ASSERT((is_same<char const volatile* const volatile* const
volatile, remove_reference<char const volatile* const volatile* const
volatile&>::type>::value));

and the second part provides library's users with a mechanism to make the
above code work not only for 'char', 'int' or other built-in type, but for
they own types too:

struct my {};
BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(my)
BOOST_STATIC_ASSERT((is_same<my, remove_reference<my&>::type>::value));
BOOST_STATIC_ASSERT((is_same<my, remove_const<my const>::type>::value));
// etc.

Note that the maco BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION
evaluates to nothing on those compilers that do support partial
specialisation.

Aleksey