c++ - 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 hold the integer parameter.

So:

template <uint64_t k>
class A {
  ??? u_;
};

For A<0>, u_ should be of type uint8_t. Same for A<255>. For A<256>, u_ should be of type uint16_t, etc.

How would you implement this?

Answer 1

This piece of metaprogramming trickery achieves it:

template<unsigned int Y> struct typed
{
    typedef typename typed<(Y & (Y - 1)) == 0 ? Y / 2 : (Y & (Y - 1))>::type type;
};

template<> struct typed<0>
{
    typedef std::uint8_t type;
};

template<> struct typed<256>
{
    typedef std::uint16_t type;
};

template<> struct typed<65536>
{
    typedef std::uint32_t type;
};

/* ToDo - add more specialisations as necessary*/

template<unsigned k> class A
{
public:
    unsigned static const k_ = k; /*constexpr if your compiler supports it*/
    typename typed<k>::type u_;
};

The usage is exactly in the question.

The unspecialised template version takes the previous type. typed<0> blocks the static recursion. The other specialisations act as anchoring points for the appropriate types.

The compile-time evaluable (Y & (Y - 1)) == 0 ? Y / 2 : (Y & (Y - 1)) reduces the number of instantiations by removing the rightmost bit of Y until a power of 2 is reached, and then divides by 2 subsequently to that. (Acknowledge @Jarod42).