mirror of
https://github.com/boostorg/hana.git
synced 2026-01-31 20:22:10 +00:00
63 lines
1.6 KiB
C++
63 lines
1.6 KiB
C++
/*
|
|
@copyright Louis Dionne 2015
|
|
Distributed under the Boost Software License, Version 1.0.
|
|
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
|
|
*/
|
|
|
|
#include <boost/hana/ext/std/utility.hpp>
|
|
#include <boost/hana/optional.hpp>
|
|
#include <boost/hana/tuple.hpp>
|
|
#include <boost/hana/type.hpp>
|
|
|
|
#include <type_traits>
|
|
using namespace boost::hana;
|
|
|
|
|
|
//! [monadic_fold.left]
|
|
auto builtin_common_t = sfinae([](auto&& t, auto&& u) -> decltype(type<
|
|
std::decay_t<decltype(true ? traits::declval(t) : traits::declval(u))>
|
|
>) { return {}; });
|
|
|
|
template <typename ...T>
|
|
struct common_type { };
|
|
|
|
template <typename T, typename U>
|
|
struct common_type<T, U>
|
|
: std::conditional_t<std::is_same<std::decay_t<T>, T>{} &&
|
|
std::is_same<std::decay_t<U>, U>{},
|
|
decltype(builtin_common_t(type<T>, type<U>)),
|
|
common_type<std::decay_t<T>, std::decay_t<U>>
|
|
>
|
|
{ };
|
|
|
|
template <typename T1, typename ...Tn>
|
|
struct common_type<T1, Tn...>
|
|
: decltype(monadic_fold<Optional>.left(
|
|
tuple_t<Tn...>,
|
|
type<std::decay_t<T1>>,
|
|
sfinae(metafunction<common_type>)
|
|
))
|
|
{ };
|
|
|
|
template <typename ...Ts>
|
|
using common_type_t = typename common_type<Ts...>::type;
|
|
|
|
static_assert(std::is_same<
|
|
common_type_t<char, short, char, short>, int
|
|
>{}, "");
|
|
|
|
static_assert(std::is_same<
|
|
common_type_t<char, double, short, char, short, double>, double
|
|
>{}, "");
|
|
|
|
static_assert(std::is_same<
|
|
common_type_t<char, short, float, short>, float
|
|
>{}, "");
|
|
|
|
static_assert(
|
|
sfinae(metafunction<common_type>)(type<int>, type<int>, type<int*>) == nothing
|
|
, "");
|
|
//! [monadic_fold.left]
|
|
|
|
int main() { }
|