Re: Template-Meta: Finding out whether a template-definition exists

From:

Barry <dhb2000@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 03 Sep 2007 21:39:42 +0800

Message-ID:

<fbh2s3$evv$1@aioe.org>

xtrigger303@gmail.com wrote:

On Sep 3, 2:47 pm, Barry <dhb2...@gmail.com> wrote:

xtrigger...@gmail.com wrote:

On Sep 3, 2:13 pm, "Hendrik Schober" <SpamT...@gmx.de> wrote:

Hi,
suppose we have
  template< typename T >
  struct X;
and some specializations:
  template<>
  struct X<A> {};
  template<>
  struct X<B> {};
  template<>
  struct X<B> {};
Given a type 'U', is there a way to find out whether the
definition 'X<U>' exists? (The result should be a compile-
time constant, so that it can be used for specializing
other templates.)
Schobi
--
SpamT...@gmx.de is never read
I'm HSchober at gmx dot de
"A patched buffer overflow doesn't mean that there's one less way attackers
can get into your system; it means that your design process was so lousy
that it permitted buffer overflows, and there are probably thousands more
lurking in your code."
Bruce Schneier

IMHO
template< typename T >
struct X
{
enum { kIsSpecialized = false };
};
template<>
struct X< A >
{
enum { kIsSpecialized = true };
};
if you cannot touch the structs you might keep a manually updated

what do you mean by "you cannot touch the structs"?

typelist ( a la Alexandrescu ) of the specialized types anche check if
a type is in the typelist...

what will the typelist contains?

I can't think of any other way ( so probably there are a thousands
more ... )

--
Thanks
Barry- Hide quoted text -

- Show quoted text -

If the structs are made by someone else and you cannot insert an
enum...

you might do something like

typedef TypeListCreator< A, B, C, D >::tResult
typeListOfClassXYZSpecializations;

then you can check if a type is in there with something like

enum { kIsTypeASpecialized = ( TypeListIndexOf<
typeListOfClassXYZSpecializations, A >::kResult != 0 ) };

actually != -1, if our versions are the same here
well, the naming convention of Loki is quite different with mine

As I write
typedef LOKI_TYPELIST_2(X<int>, X<float>) SpecializedList;
BOOST_STATIC_ASSERT((Loki::TL::IndexOf<SpecializedList, X<bool>
>::value) == -1);

TypeListCreator and TypeListIndexOf are typelist handling classes a la
Alexandrescu.

Obviously the list must be manually updated so it is helpful just to
keep everything in one place...

Regards,
FB

--
Thanks
Barry