Lately i tried to write wrapper arount void_t idiom simple as following:
namespace detail {
template <class Traits, class = void>
struct applicable : std::false_type {};
template <class... Traits>
struct applicable<std::tuple<Traits...>, std::void_t<Traits...>>
: std::true_type {};
} // namespace detail
template <class... Traits>
using applicable = detail::applicable<Traits...>;
and something like than on the call side
template <class T>
using call_foo = decltype(std::declval<T>().foo());
template <class T>
using has_foo = applicable<call_foo<T>>;
auto main() -> int {
std::cout << std::boolalpha << has_foo<std::vector<int>>::value;
}
but insted of expected "false" i get compile-time error:
error: 'class std::vector<int, std::allocator<int> >' has no member named 'foo'
using has_foo = my::applicable<call_foo<T>>;
What is wrong?
Update: The idea behind using parameter pack in traits is to use this applicable metafunction as follows:
template <class T>
using call_foo = decltype(std::declval<T>().foo());
template <class T>
using call_boo = decltype(std::declval<T>().boo());
template <class T>
using call_bar = decltype(std::declval<T>().bar());
template <class T>
using has_foo_and_boo_and_bar = applicable<call_foo<T>, call_boo<T>, call_bar<T>>;
The key here is not to apply trait onto multiple classes but to apply multiple traits onto one class without the use of std::conjunction.
Something like that :
#include <type_traits>
#include <vector>
using namespace std;
struct Foo{
int foo() { return 1 ;}
};
// transform an "maybe type" into a classic type traite
// We use T and not Args... so we can have a default type at the end
// we can use a type container (like tuple) but it need some extra boilerplate
template<template<class...> class Traits, class T, class = void>
struct applicable : std::false_type {};
template<template<class...> class Traits, class T>
struct applicable<
Traits,
T,
std::void_t< Traits<T> >
> : std::true_type {};
// not an usual type trait, I will call this a "maybe type"
template <class T>
using call_foo = decltype(std::declval<T>().foo());
// creating a type trait with a maybe type
template<class T>
using has_foo_one = applicable<call_foo, T>;
static_assert( has_foo_one<std::vector<int>>::value == false );
static_assert( has_foo_one<Foo>::value == true );
// we need a way to check multiple type at once
template <
template<class...> class Traits,
class... Args
>
inline constexpr bool all_applicable = (applicable<Traits,Args>::value && ...);
static_assert( all_applicable<call_foo,Foo,Foo> == true );
static_assert( all_applicable<call_foo,Foo,int> == false );
template<class ... Args>
struct List{};
// if you want the exact same syntaxe
template<
template<class...> class Traits, // the type traits
class List, // the extra boilerplate for transforming args... into a single class
class = void // classic SFINAE
>
struct applicable_variadic : std::false_type {};
template<
template<class...> class Traits,
class... Args
>
struct applicable_variadic
<Traits,
List<Args...>, // can be std::tuple, or std::void_t but have to match line "using has_foo..."
std::enable_if_t<all_applicable<Traits, Args...> // will be "void" if all args match Traits
>
> : std::true_type {};
template<class... Args>
using has_foo = applicable_variadic<call_foo, List<Args...>>;
static_assert( has_foo<Foo,Foo>::value == true );
static_assert( has_foo<Foo>::value == true );
static_assert( has_foo<Foo,int>::value == false );
int main() {
return 1;
}
https://godbolt.org/z/rzqY7G9ed
You probably can write all in one go, but I separate each part. I found it easier to understand when I go back on my code later on.
Note:
In your update you want:
template <class T>
using call_foo = decltype(std::declval<T>().foo());
template <class T>
using call_boo = decltype(std::declval<T>().boo());
template <class T>
using call_bar = decltype(std::declval<T>().bar());
template <class T>
using has_foo_and_boo_and_bar = applicable<call_foo<T>, call_boo<T>, call_bar<T>>;
It's impossible. applicable<int, ERROR_TYPE> will not compile. It's not a "substitution error" it is an error.
You have 2 options (AFAIK)
applicable<traits_foo<T>::value, traits_bar<T>::value>. Note the value. In this case each type trait will tell if a property is respected and applicable will just check that all boolean are true.type_traits<T> but just type_traits) and the type to check and use SFINAE in applicable. That what I have done below.With the same principle, we can create a "list of template class". On this implementation, we expect a type traits to have a ::value that why I pass has_bar_one and not call_bar
template<template<class...> class... Traits>
struct list_of_template_class{};
template<
class ListOfTraits,
class T,
class = void
>
struct applicable_X_traits : std::false_type {};
template<
template<class...> class... Traits ,
class T
>
struct applicable_X_traits
<list_of_template_class<Traits...>,
T,
std::enable_if_t< ( Traits<T>::value && ...) >
> : std::true_type {};
template <class T>
using call_bar = decltype(std::declval<T>().foo());
template<class T>
using has_bar_one = applicable<call_foo, T>;
template<class T>
using has_foo_bar = applicable_X_traits<
list_of_template_class<has_bar_one, has_foo_one>,
T
>;
static_assert(has_foo_bar<Foo>::value == true );
static_assert(has_foo_bar<int>::value == false );
struct JustBar {
void bar() { }
};
static_assert(has_foo_bar<JustBar>::value == false );
https://godbolt.org/z/K77o3KxTj
Or just use Boost::Hana
// If you have an instance of T you can just do :
auto has_foo_bar_simpler = hana::is_valid([](auto&& p) -> std::void_t<
decltype(p.foo()),
decltype(p.bar())
>{ });
static_assert(has_foo_bar_simpler(1) == false );
static_assert(has_foo_bar_simpler(JustBar{}) == false );
static_assert(has_foo_bar_simpler(Foo{}) == true );
// if not
template<class T>
constexpr bool has_foo_bar_simpler2 = decltype(has_foo_bar_simpler(std::declval<T>())){};
static_assert(has_foo_bar_simpler2<int> == false );
static_assert(has_foo_bar_simpler2<JustBar> == false );
static_assert(has_foo_bar_simpler2<Foo> == true );