Re: SFINAE and non member functions detection.
On 12/8/2010 12:56, german diago wrote:
Hello. I'm trying to detect wether a non-member function exists for a
given type.
I can do the same piece of code work for member functions, but for non-
member I'm having trouble,
because when using decltype (funcIwantToDetect(t)) it fails if the
function does not exist.
Can anyone give me another way to accomplish the same thing? I tried
with this code for now:
#define DEFINE_HAS_FREE_FUNCTION_TRAIT(name) \
extern struct hasFreeFunction##name##__ { \
template<class T, class U = decltype (name(Type<T>()))> \
std::true_type operator()(Type<T>, U * = 0); \
template<class T> \
std::false_type operator()(T); \
} hasFreeFunction##name##_; \
template<class T, class U = decltype(hasFreeFunction##name##_(T()))>
\
struct HasFreeFunction##name : public U {}
Thanks for your time.
It would have been a good idea, if you would have resolved the used entities like the Type template for us. Without this information there are several reasons of failure. It is also not clear to me, what the exact aim of the trait is. Do you want that the trait returns
true in the following situation:
DEFINE_HAS_FREE_FUNCTION_TRAIT(bar);
void bar(double);
static_assert(HasFreeFunctionbar<int>::value, "Ouch"); // OK?
It would return true, because int is convertible to double. I believe that your originally intended approach would behave similarly. But is that intended?
Here is one possible approach to solve the problem you seem to intend to solve (quick & dirty, not something to be proud of):
#include <type_traits>
#include <utility>
template<class> struct Type{};
struct never{};
#define DEFINE_HAS_FREE_FUNCTION_TRAIT(name) \
extern void name(never);\
template<class T>\
struct hasFreeFunction##name##__ { \
template <class U, class = decltype(name(std::declval<U>()))> \
static std::true_type test(Type<U>*); \
template <class> \
static std::false_type test(...); \
typedef decltype(hasFreeFunction##name##__::test<T>(0)) result;\
}; \
template <class T> \
struct HasFreeFunction##name : hasFreeFunction##name##__<T>::result {}
HTH & Greetings from Bremen,
Daniel Kr?gler
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