Question

I have a template class that accepts from 1 to 8 integer arguments. The permitted range for each argument is 0..15. A default value of 16 for each argument allows me to detect unused arguments.

I would like to have the number of user-supplied arguments available as a compile-time constant. I can do this using a template helper class and a lot of partial specialisation.

My question is, can I clean this up using a bit of recursive metaprogramming. What I have works but it feels like it could be improved syntactically.

Variadic templates and anything else c++0x are not available to me, sadly.

#include <stdint.h>
#include <iostream>

template<uint8_t p0,uint8_t p1,uint8_t p2,uint8_t p3,uint8_t p4,uint8_t p5,uint8_t p6,uint8_t p7>
struct Counter { enum { COUNT=8 }; };

template<uint8_t p0,uint8_t p1,uint8_t p2,uint8_t p3,uint8_t p4,uint8_t p5,uint8_t p6>
struct Counter<p0,p1,p2,p3,p4,p5,p6,16> { enum { COUNT=7 }; };

template<uint8_t p0,uint8_t p1,uint8_t p2,uint8_t p3,uint8_t p4,uint8_t p5>
struct Counter<p0,p1,p2,p3,p4,p5,16,16> { enum { COUNT=6 }; };

template<uint8_t p0,uint8_t p1,uint8_t p2,uint8_t p3,uint8_t p4>
struct Counter<p0,p1,p2,p3,p4,16,16,16> { enum { COUNT=5 }; };

template<uint8_t p0,uint8_t p1,uint8_t p2,uint8_t p3>
struct Counter<p0,p1,p2,p3,16,16,16,16> { enum { COUNT=4 }; };

template<uint8_t p0,uint8_t p1,uint8_t p2>
struct Counter<p0,p1,p2,16,16,16,16,16> { enum { COUNT=3 }; };

template<uint8_t p0,uint8_t p1>
struct Counter<p0,p1,16,16,16,16,16,16> { enum { COUNT=2 }; };

template<uint8_t p0>
struct Counter<p0,16,16,16,16,16,16,16> { enum { COUNT=1 }; };


template<uint8_t p0,uint8_t p1=16,uint8_t p2=16,uint8_t p3=16,
         uint8_t p4=16,uint8_t p5=16,uint8_t p6=16,uint8_t p7=16>
struct MyClass {

  void printArgCount() {
    std::cout << Counter<p0,p1,p2,p3,p4,p5,p6,p7>::COUNT << std::endl;
  }
};


main() {
  MyClass<4,7,8,12,15,1> foo;

  foo.printArgCount();
}

Answer 1

Why not just checking the first 16 that appears ?

template <uint8_t p0, ...>
struct Counter {
    enum { COUNT = (p1 == 16 ? 1 : 
                    p2 == 16 ? 2 :
                    p3 == 16 ? 3 :
                    p4 == 16 ? 4 :
                    p5 == 16 ? 5 :
                    p6 == 16 ? 6 :
                    p7 == 16 ? 7 :
                               8
                   ) };
};