C++ 中文周刊 第109期

class integer {
public:
    template<typename T, typename = std::enable_if_v<std::is_arithmetic_v<T>>>
    explicit(std::is_floating_point_v<T>)
    integer(T t) : val(std::round(t)) {}
private:
    int val;
};
void func(integer i) {/*...*/}
{
    // func(3.4); // won't compile
    func(integer(3.4));
    func(5);
}

#include <algorithm>
#include <array>
#include <cstdint>
#include <string_view>
#include <utility>

// clang-format off
template <std::size_t N>
struct fixed_string final {
    constexpr explicit(false) fixed_string(const char (&str)[N + 1]) {
        std::copy_n(str, N + 1, std::data(data));
    }

    [[nodiscard]] constexpr auto operator<=>(const fixed_string&) const =
        default;

    std::array<char, N + 1> data{};
};

template <std::size_t N> fixed_string(const char (&str)[N]) -> fixed_string<N - 1>;

template <fixed_string>
struct types final {
    friend auto get(types);
    template <class T>
    struct set {
        friend auto get(types) { return T{}; }
    };
};

template <fixed_string Str>  // typename erasure
struct te : decltype(get(types<Str>{})) {};

template<fixed_string Str, class T>
[[nodiscard]] constexpr auto typename_cast(const T& t) {
  void(typename types<Str>::template set<T>{});
  return static_cast<te<Str>>(t);
}

constexpr auto show_types(auto...) -> void;

template <class...> struct foo {};
template <class... Ts>
auto fn() -> void {
    foo<Ts...> foo{};
    show_types(typename_cast<"foo.long">(foo)); // tefoo.long instead of foo<main()::lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type, ...>
}

int main() {
    struct lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type {};

    fn<
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type,
        lllllllllllllllllllllllllllllllllllllllllllllllllllllllllong_type
    >();
}

// clang-format on

char table1(int idx) {
  const char array[] = {'z', 'b', 'k', 'd'};
  return array[idx];
}

std::string table2(int idx) {
  const std::string array[] = {"a", "l", "a", "z"};
  return array[idx];
}

std::string table3(int idx) {
  const static std::string array[] = {"a", "l", "a", "z"};
  return array[idx];
}

std::string_view table4(int idx) {
  constexpr static std::string_view array[] = {"a", "l", "a", "z"};
  return array[idx];
}