Re: [Boost-Users] Boost Concept Check Library

21 Aug 2002

      Hi Jeremy,

--- Jeremy Siek <jsiek@cs.indiana.edu> wrote:
...
Hi Igor,
What about concepts defined within a doubly nested
namespace? Will that
also work?
You're right. There is a problem in my solution caused
by using 'ns' macro parameter in constructing names.
So, we could sacrifice the uniqueness of names to
usability. 
The fixed solution looks like following:

[code

#define BOOST_CLASS_REQUIRES(type_var, ns, concept) \
  typedef void (ns::concept <type_var>::*
func##type_var##concept)(); \
  template <func##type_var##concept _Tp1> \
  struct concept_checking_##type_var##concept { }; \
  typedef concept_checking_##type_var##concept< \
    BOOST_FPTR ns::concept <type_var>::constraints> \
    concept_checking_typedef_##type_var##concept

end code]

Actually, it was the first version I tried.
Now it will work for nested namespaces:

[code

namespace ns1
{
  namespace ns2
  {
    template <class TT>
    class DummyConcept
    {
      void constraints() { /* dummy */}
    };
  }
}

template <class T>
class X
{
  BOOST_CLASS_REQUIRES(T, ns1::ns2, DummyConcept);
};

int main(int, char**) 
{
  X<int> x1;
  return 0;
}

end code]
...
Have you tested the code below with VC++?
Yes, I tested it with VC++, SP5.

Actually, VC++ is very freakish in concern to
templates.
Let me give an example that expose one of the strange
behaviours of VC++ that couldn't be understood from 
the point of view of common sense:

[code

template <class TT>
class A 
{ 
public:
  void foo() {}
};

template <class T, class V>
class X
{
  typedef void (A<T>::*fp)();
  template <fp _Tp1> struct ss { };
  typedef ss<&A<T>::foo> tt;
};

int main(int, char**) 
{
  X<int, float> x1;
  return 0;
}

end code]

The above example will compile, but the following
won't:

[code

template <class TT>
class A 
{ 
public:
  void foo() {}
};

template <class T, class V>
class X
{
  typedef void (A<V>::*fp)();
  template <fp _Tp1> struct ss { };
  typedef ss<&A<V>::foo> tt;
};

int main(int, char**) 
{
  X<int, float> x1;
  return 0;
}

end code]

And I'm not surprised, because I have compiled more
than 20 deficiencies of VC++ compiler mostly concerned
C++ templates support.

Best regards,
     Igor Bashkirov.
...
On Tue, 20 Aug 2002, igor_bashkirov wrote:
igor_b>
igor_b> [code
igor_b>
igor_b> #define BOOST_CLASS_REQUIRES(type_var, ns,
concept) \
igor_b>   typedef void (ns::concept <type_var>::*
func##type_var##ns##concept)
igor_b> (); \
igor_b>   template <func##type_var##ns##concept
_Tp1> \
igor_b>   struct
concept_checking_##type_var##ns##concept { }; \
igor_b>   typedef
concept_checking_##type_var##ns##concept< \
igor_b>     BOOST_FPTR ns::concept
<type_var>::constraints> \
igor_b>    
concept_checking_typedef_##type_var##ns##concept;
igor_b>
igor_b> end code]
----------------------------------------------------------------------
...
Jeremy Siek                         
http://php.indiana.edu/~jsiek/
 Ph.D. Student, Indiana Univ. B'ton   email:
jsiek@osl.iu.edu
 C++ Booster (http://www.boost.org)   office phone:
(812) 855-3608
----------------------------------------------------------------------
...
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