I have a template that takes a template template parameter and a pack of type arguments. I want to instantiate the template with the arguments only if the arities match. Something like this:
// can_apply_t = ???
template<template<typename...> typename Template, typename... Args>
struct test {
using type = std::conditional_t<can_apply_t<Template, Args...>, Template<Args...>, void>;
};
template<typename> struct unary_template;
template<typename, typename> struct binary_template;
static_assert(std::is_same_v< test<unary_template, int>::type, unary_template<int> >);
static_assert(std::is_same_v< test<binary_template, int>::type, void >);
I fantasized that it would be as simple as this:
template<template<typename... T> typename Template, typename... Args>
struct test {
using type = std::conditional_t<sizeof...(T) == sizeof...(Args), Template<Args...>, void>;
};
...but clang 12 says:
error: 'T' does not refer to the name of a parameter pack
CodePudding user response:
fantasized that it would be as simple as this:
No... not as simple... when you write
using type = std::conditional_t<can_apply_t<Template, Args...>,
Template<Args...>,
void>;
the Template<Args...>
must be an acceptable type also when test of conditional_t
is false.
You need a can_apply_t
that directly return Template<Args...>
when possible, void
otherwise.
I propose something as the following
template <template <typename...> class C, typename... Ts>
auto can_apply_f (int)
-> std::remove_reference_t<decltype(std::declval<C<Ts...>>())>;
template <template <typename...> class C, typename... Ts>
auto can_apply_f (long) -> void;
template <template <typename...> class C, typename ... Ts>
using can_apply_t = decltype(can_apply_f<C, Ts...>(0));
template <template<typename...> typename Template, typename... Args>
struct test {
using type = can_apply_t<Template, Args...>;
};
CodePudding user response:
If you can use C 20, concepts will make things much easier.
template<template<typename...> typename, typename...>
struct test {
using type = void;
};
template<template<typename...> typename Template, typename... Args>
requires requires { typename Template<Args...>; }
struct test<Template, Args...> {
using type = Template<Args...>;
};