Value type deduction from non-type template parameter

Question

I'm currently trying to implement a small template which deduces the type needed to store a certain number of bits given as template parameter:

template<unsigned char BITS> class Register { public: unsigned long type; };

Furthermore I'm trying to specialize this template for certain bit sizes:

template<> class Register<8> { public: unsigned char type; }; template<> class Register<16> { public: unsigned short type; }; template<unsigned int N> Register<N+1>;

Unfortunately this doesn't work as intended and fails to compile:

int _tmain(int argc, _TCHAR* argv[]) { Register<32>::type val32 = 0xDEADBEEF; assert(sizeof(val) == sizeof(unsigned long) ); Register<16>::valType val16 = 0xBEEF; assert(sizeof(val) == sizeof(unsigned short) ); Register<8>::valType val8 = 0xEF; assert(sizeof(val) == sizeof(unsigned char) ); Register<4>::valType val4 = 0xF; assert(sizeof(val) == sizeof(unsigned char) ); return 0; }

Maybe someone can give me a pointer to some helpful text or tell me what's wrong with my approach?

Kerrek SB · Accepted Answer · 2016-04-22 10:44:02Z

5

You want a type member, not a data member:

template <std::size_t N> struct Register { using type = unsigned long int; }; template <> struct Register<8> { using type = unsigned char; }; // ...

Usage:

template <typename T> void f() { typename Register<sizeof(T) * CHAR_BIT>::type thing; // ... }

answered Apr 22, 2016 at 10:44

Kerrek SB

480k96 gold badges904 silver badges1.1k bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

fhw72 Over a year ago

Thanks. I totally overlooked that. Can this template be extended to all other parameter values (1-32) in a generic way? That means that e.g. Register<17> automatically selects the Register<32> declaration?

Kerrek SB Over a year ago

@fhw72: Sure. You can have some intermediate dispatching mechanism that "snaps" to the nearest implemented value, etc.

fhw72 Over a year ago

Can this also be achieved at compile time? Like Register<N> uses Register<N+1>.... and so on and eventually "snaps" in at the next explicit defined template?

Kerrek SB Over a year ago

Yes, you can compute "N+1" at compile time, and also things like "(N+7)/8*N" etc.

fhw72 Over a year ago

Could you please give me an example? I don't get it currently. BTW: I thought in this "recursive" direction: template<unsigned char BITS> struct Register { using type = Register<BITS +1>::type; };

|

Collectives™ on Stack Overflow

Value type deduction from non-type template parameter

1 Answer 1

8 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

8 Comments

Related