Select an integer type based on template integer parameter

Question

I would like to create a class template which takes an unsigned integer parameter and has a member u_ whose type is the smallest unsigned integer type that will

Answer 1

template
struct A;

template<>
struct A<0>{
    enum {id = 0};
    using type = uint8_t;
};

template<>
struct A<255>{
    enum {id = 255};
    using type = uint8_t;
};

template<>
struct A<256>{
    enum {id = 256};
    using type = uint16_t;
};

int main(){
    typename A<255>::type a0 = 255; // uint8_t
    typename A<256>::type a1 = 256; // uint16_t
}

But I think you want to have a class so that every value under 256 will define uint8_t, every value under 65536 will define uint16_t etc. etc.

So just incase, this is how you would do that:

template
struct A{
    enum {value = Value};
    using type = std::conditional_t<
        (Value <= std::numeric_limits::max()), uint8_t,
            std::conditional_t<
                (Value <= std::numeric_limits::max()), uint16_t,
                    std::conditional_t<
                        (Value <= std::numeric_limits::max()), uint32_t
                            std::conditional_t<
                                (Value <= std::numeric_limits::max()), uint64_t, uintmax_t
                            >
                    >
            >
    >;
};

And here's a live example