Re: [Boost-users] Detecting Valid Expressions for a Type at Compile-Time

Martin Ecker writes:

Hi,

Is there a way, using SFINAE tricks and potentially the techniques used in the Boost Concept Check Library, to detect at compile-time if a type T supports a certain expression? In my case, I'm interested if T supports operator ==. So I'm basically looking for a compile-time predicate so that this compiles:

struct my_struct { int i; }; struct my_struct2 { int i; }; bool operator == (my_struct2 const&, my_struct2 const&) const;

BOOST_STATIC_ASSERT((supports_comparison_operator<int>::value)); BOOST_STATIC_ASSERT((!supports_comparison_operator::value)); BOOST_STATIC_ASSERT((supports_comparison_operator::value));

This implementation is based on boost/detail/is_incrementable.hpp. #include #include namespace detail { struct tag {}; struct any { template <class T> any(T const&); }; tag operator==(any const&, any const&); tag operator,(tag,int); char (& check(tag))[2]; template <class T> char check(T const&); template <class T> struct impl { static typename boost::remove_cv<T>::type& x; static const bool value = sizeof(check(((x == x),0))) == 1; }; } template <class T> struct is_eq_comparable : detail::impl<T> {}; struct A{}; int main() { BOOST_STATIC_ASSERT(is_eq_comparable<int>::value); BOOST_STATIC_ASSERT(!is_eq_comparable<A>::value); } Roman Perepelitsa.