Angus Leeming wrote in news:bvqt8a$6dq$1@titan.btinternet.com:
[color=blue]
> I'm trying to write some class templates to ascertain whether a given
> class defines a given member type.
>
> I have been using SFINAE and the "sizeof trick" to do so and, in
> general, all works well. The code snippet, below, demonstrates my
> problem however. The test fails when I try and ascertain whether the
> class defines a "reference" member type because I cannot declare a
> pointer to a reference. Uncommenting the penultimate line of code
> will result in the compiler telling me just that. Fair enough and
> understood.
>
> My question is simply "how should I proceed"?
>[/color]
#include <iostream>
#include <ostream>
#include <vector>
typedef char One;
typedef struct { char a[2]; } Two;
/* Could do with a better name :)
*/
template < typename Valid > struct has
{
typedef One type;
};
/* These 4 function's have bodies just so the last line of main
compiles and run's (i.e for testing).
*/
template <typename T>
typename has< typename T::value_type >::type has_value_type( int )
{
return One();
}
template <typename T>
Two has_value_type(...)
{
return Two();
}
template <typename T>
typename has< typename T::reference >::type has_reference( int )
{
return One();
}
template <typename T>
Two has_reference(...)
{
return Two();
}
int main()
{
typedef std::vector<int> IntVec;
std::cout << "IntVec has value_type? "
<< (sizeof(has_value_type<IntVec>(0)) == 1)
<< std::endl;
std::cout << "IntVec has reference? "
<< (sizeof(has_reference<IntVec>(0)) == 1)
<< std::endl;
std::cout << "int has reference? "
<< (sizeof(has_reference< int >(0)) == 1)
<< std::endl;
has_value_type<IntVec>(0);
return 0;
}
HTH.
Rob.
--
http://www.victim-prime.dsl.pipex.com/ 
Ascertaining whether T::reference exists. 
