#include <type_traits>
#define str_cat(first, second) first##second
#define has_xxx(member_name) \
template<class T, class U = void> \
struct str_cat(has_, member_name): std::false_type {};\
template<class T>\
struct str_cat(has_, member_name)<T, typename SFINAE<typename T::member_name>::type>\
: std::true_type {};\
template<class T>\
struct str_cat(has_, member_name)<T, typename SFINAE<decltype(T::member_name)>::type>\
: std::true_type {};
template<class T>
struct SFINAE {
using type = void;
};
struct A {
int i = 0;
void foo() {}
using void_t = void;
};
struct B {
int j = 0;
void goo() {}
using void_t = void;
};
has_xxx(i)
has_xxx(foo)
has_xxx(j)
has_xxx(goo)
has_xxx(void_t) //compile error if `has_xxx(i)` appears at the head
int main()
{
//has_i<A>::value; // true
//has_i<B>::value; // false
has_foo<A>::value; // true
has_foo<B>::value; // false
has_goo<B>::value; // true
has_void_t<A>::value; // true
has_void_t<B>::value; // true
return 0;
}
On VS2017 it fails to compile
https://gcc.godbolt.org/z/JkOhLi
error: C2752 'template' : more than one partial specialization matches the template argument list.
But it's ok on gcc and clang. http://coliru.stacked-crooked.com/a/6b9490f6b127ae88
If I change the order of the macro, it compiles:
has_xxx(foo)
has_xxx(i) //now compiles successfully
has_xxx(j)
has_xxx(goo)
has_xxx(void_t) //compile error if `has_xxx(i)` appears at the head
Or just change the name of member in struct A:
struct A {
int k = 0; // i -> k, now compiles successfully !!!!
void foo() {}
using void_t = void;
};
I can't figure out the reason.
Does order of macro matter or it's a bug of MSVC on SFINAE?
It looks like a MSVC bug, however it can be easily avoided like this:
#include <type_traits>
#define str_cat(first, second) first##second
#define custom_trait(trait_name, expr) \
template<class T, class U = void> \
struct trait_name: std::false_type {}; \
template<class T> \
struct trait_name<T, std::void_t<expr>> : std::true_type {};
#define has_xxx(member_name) \
custom_trait(str_cat(has_type_, member_name), typename T::member_name) \
custom_trait(str_cat(has_value_, member_name), decltype(T::member_name)) \
template<class T>\
using str_cat(has_, member_name) = \
std::bool_constant<str_cat(has_type_, member_name)<T>::value \
|| str_cat(has_value_, member_name)<T>::value>;
Note : this code (as well as yours) does not let you detect methods.
I suggest you to report the issue ( Help -> Send Feedback -> Report a problem
is Visual Studio).
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