c++ - Select an integer type based on template integer parameter -

i create class template takes unsigned integer parameter , has member u_ type smallest unsigned integer type hold integer parameter.

so:

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

for a<0>, u_ should of type uint8_t. same a<255>. a<256>, u_ should of type uint16_t, etc.

how implement this?

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

the usage in question.

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

the compile-time evaluable (y & (y - 1)) == 0 ? y / 2 : (y & (y - 1)) reduces number of instantiations removing rightmost bit of y until power of 2 reached, , divides 2 subsequently that. (acknowledge @jarod42).