#include <cstdint>
#include <string>
#include <type_traits>
// values_equal<a, b, T>::value is true if and only if a == b.
template<auto V1, decltype(V1) V2, typename T>
struct values_equal : std::bool_constant<V1 == V2>
{
using type = T;
};
// default_type<T>::value is always true
template<typename T>
struct default_type : std::true_type
{
using type = T;
};
// Now we can use disjunction like a switch statement:
template<int I>
using int_of_size = typename std::disjunction< //
values_equal<I, 1, std::int8_t>, //
values_equal<I, 2, std::int16_t>, //
values_equal<I, 4, std::int32_t>, //
values_equal<I, 8, std::int64_t>, //
default_type<void> // must be last!
>::type;
static_assert(sizeof(int_of_size<1>) == 1);
static_assert(sizeof(int_of_size<2>) == 2);
static_assert(sizeof(int_of_size<4>) == 4);
static_assert(sizeof(int_of_size<8>) == 8);
static_assert(std::is_same_v<int_of_size<13>, void>);
// checking if Foo is constructible from double will cause a hard error
struct Foo
{
template<class T>
struct sfinae_unfriendly_check { static_assert(!std::is_same_v<T, double>); };
template<class T>
Foo(T, sfinae_unfriendly_check<T> = {});
};
template<class... Ts>
struct first_constructible
{
template<class T, class...Args>
struct is_constructible_x : std::is_constructible<T, Args...>
{
using type = T;
};
struct fallback
{
static constexpr bool value = true;
using type = void; // type to return if nothing is found
};
template<class... Args>
using with = typename std::disjunction<is_constructible_x<Ts, Args...>...,
fallback>::type;
};
int main() {
// OK, is_constructible<Foo, double> not instantiated
static_assert(std::is_same_v<first_constructible<std::string, int, Foo>::with<double>,
int>);
static_assert(std::is_same_v<first_constructible<std::string, int>::with<>, std::string>);
static_assert(std::is_same_v<first_constructible<std::string, int>::with<const char*>,
std::string>);
static_assert(std::is_same_v<first_constructible<std::string, int>::with<void*>, void>);
}
标签:std,有意思,int,assert,static,template,disjunction,type
From: https://www.cnblogs.com/DesertCactus/p/18422219