Re: [boost] [type_traits] adding is_less_comparable<T>, etc.

Subject: Re: [boost] [type_traits] adding is_less_comparable<T>, etc.
From: Frédéric Bron (frederic.bron_at_[hidden])
Date: 2009-09-02 04:52:32

Thanks for all contributions. It works very fine now.
1. the key is the use of the ... argument instead of a template parameter
2. there is no difference between using a factory function and using a
static reference to a value of the type -> I prefer the static
reference
3. there is no difference between using pass/fail and return char or
char[2] but pass/fail is easier to read -> I keep it

So the following code has the following properties:
* returns true if t<u is meaningful and returns a value convertible to bool
* returns false if t<u is meaningless.
* fails with compile time error if t<u is meaningful and returns void

If you agree with the implementation, I will modify the sandbox
accordingly and propose a new submission.
Can I add your names in the copyright notice ? (Robert Stewart, Steven
Watanabe, Roman Perepelitsa).

Frédéric

#include <iostream>
#include <boost/type_traits/remove_cv.hpp>
#include <boost/type_traits/integral_constant.hpp>

namespace boost {
namespace detail {

// This namespace ensures that ADL does not mess things up.
namespace is_less_comparable_impl {

// a type returned from comparison operator when no such operator is
found in the
// type's own namespace
struct tag { } ;

// any soaks up implicit conversions and makes the following
// comparison operators less-preferred than any other such operators that
// might be found via ADL.
struct any { template <class T> any(T const&) ; } ;

// when operator< is not available, this one is used
tag operator<(const any&, const any&) ;

typedef char pass ;
struct fail { char dummy[2] ; } ;

pass is_bool(bool) ; // this version is preferred for types convertible to bool
fail is_bool(...) ; // this version is used otherwise

template < typename T, typename U >
struct is_less_comparable_impl {
        static typename boost::remove_cv<T>::type &t ;
        static typename boost::remove_cv<U>::type &u ;
        static const bool value = sizeof(is_bool(t<u)) == sizeof(pass) ;
} ;

} // namespace impl
} // namespace detail

template< typename T, typename U=T >
struct is_less_comparable :
boost::integral_constant<bool,(::boost::detail::is_less_comparable_impl::is_less_comparable_impl<T,U>::value)>
{ } ;

} // namespace boost

// for testing
struct NotComparable { } ;
struct Comparable { bool operator<(const Comparable&) const ; } ;
struct IntComparable { int operator<(const IntComparable&) ; } ;
struct PtrComparable { void *operator<(const PtrComparable&) ; } ;
// this one has a strange operator<: it returns a non standard type
convertible to bool
struct Convertible { operator bool () const ; } ;
struct StrangeComparable { Convertible operator<(const StrangeComparable&) ; } ;
struct VoidComparable { void operator<(const VoidComparable&) ; } ;

#define TEST1(T)\
        std::cout << boost::is_less_comparable<T, T>::value << "\t(" << #T <<
'<' << #T << ")\n"

#define TEST2(T1,T2)\
        std::cout << boost::is_less_comparable<T1, T2>::value << "\t(" << #T1
<< '<' << #T2 << ")\n" ;\
        std::cout << boost::is_less_comparable<T2, T1>::value << "\t(" << #T2
<< '<' << #T1 << ")\n"

int main() {
        std::cout << std::boolalpha ;

        TEST1(NotComparable) ;
        TEST1(Comparable) ;
        TEST1(IntComparable) ;
        TEST1(PtrComparable) ;
        TEST1(StrangeComparable) ;
        // the following line triggers a compile time error
        //TEST1(VoidComparable) ;

        TEST2(double, NotComparable) ;
        TEST2(int, NotComparable) ;
        TEST2(double, IntComparable) ;
        TEST2(int, IntComparable) ;
        TEST2(double, Comparable) ;
        TEST2(int, Comparable) ;

        TEST2(int, char) ;
        TEST2(int, short) ;
        TEST2(int, long) ;
        TEST2(int, float) ;
        TEST2(int, double) ;
        return 0 ;
}