Alternatives to many nested std::conditional_t?

user_700825 0
Viewed 235

I find many nested std::conditional_t hard to read so I choose a different pattern(of calling decltype on function with auto return type):

template<bool is_signed, std::size_t has_sizeof>
auto find_int_type(){
    static_assert(sizeof(int)==4);
    if constexpr(is_signed){
        if constexpr(has_sizeof==4){
            return int{};
        } else if constexpr (has_sizeof==8){
            return std::int64_t{};
        } else {
            return;
        }
    } else {
        if constexpr(has_sizeof==4){
            return (unsigned int){};
        }
        else if constexpr (has_sizeof==8){
            return std::uint64_t{};
        } else {
            return;
        }
    } 
}

static_assert(std::is_same_v<int, decltype(find_int_type<true, 4>())>);
static_assert(std::is_same_v<unsigned int, decltype(find_int_type<false, 4>())>);
static_assert(std::is_same_v<void, decltype(find_int_type<false, 3>())>);
static_assert(std::is_same_v<void, decltype(find_int_type<false, 5>())>);
static_assert(std::is_same_v<std::int64_t, decltype(find_int_type<true, 8>())>);
static_assert(std::is_same_v<std::uint64_t, decltype(find_int_type<false, 8>())>);
static_assert(std::is_same_v<void, decltype(find_int_type<false, 9>())>);

My questions are:

Is there a better way?

Is this way slower to compile than std::conditional_t(assuming types that I need to instantiate are much more expansive than in this example where I just use built in types).

P.S. this is a toy example, IRCode I would be dealing with some more complicated types.

if-constexpr typetraits templates c++
2 Answers

Personally, I feel the the clearest approach here is "data driven". Putting the criteria in a table (written as specializations of a class template) and letting the compiler do pattern matching to determine the type is shorter, less error prone and easier to read or extend.

template<bool is_signed, std::size_t has_sizeof>
struct find_int_type_impl { using type = void; }; // Default case

template<> struct find_int_type_impl<true,  4> { using type = std::int32_t;  };
template<> struct find_int_type_impl<true,  8> { using type = std::int64_t;  };
template<> struct find_int_type_impl<false, 4> { using type = std::uint32_t; };
template<> struct find_int_type_impl<false, 8> { using type = std::uint64_t; };

template<bool is_signed, std::size_t has_sizeof>
using find_int_type = typename find_int_type_impl<is_signed, has_sizeof>::type;
user_8176430

Since std::disjunction<Args...> inherits from the first type in Args... whose value is true, or if none such type exists, the last type in Args..., we can (ab)use it to produce a multiway branch:

template<class... Args>
using select = typename std::disjunction<Args...>::type;

template<bool V, class T>
struct when {
    static constexpr bool value = V;
    using type = T;
};

template<bool is_signed, std::size_t has_sizeof>
using find_int_type = select<
    when<is_signed, select<
        when<has_sizeof==4, int>,
        when<has_sizeof==8, std::int64_t>,
        when<false, void>
    >>,
    when<!is_signed, select<
        when<has_sizeof==4, unsigned int>,
        when<has_sizeof==8, std::uint64_t>,
        when<false, void>
    >>
>;
user_46725880
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