Check if one set of types is a subset of the other

Question

How can one check if one parameter pack (interpreted as a set) is a subset of another one?

So far I only have the frame (using std::tuple), but no functionality.

Answer 1

Not exactly what you asked but... just for fun, using std::is_base_of you can create (in C++14, at least) a constexpr function that works like your struct.

The following is a working example (only C++14)

#include 
#include 
#include 

template 
struct foo : std::tuple...
 { };

template 
bool isSubsetOf (std::tuple const &, std::tuple const &)
 {
   bool ret { true };

   using un = int[];
   using d2 = foo;

   (void)un { (ret &= std::is_base_of, d2>::value, 0)... };

   return ret;
 }


int main()
 {
    using t1 = std::tuple;
    using t2 = std::tuple;
    using t3 = std::tuple;

    std::cout << isSubsetOf(t1{}, t1{}) << std::endl;  // print 1
    std::cout << isSubsetOf(t1{}, t2{}) << std::endl;  // print 1
    std::cout << isSubsetOf(t2{}, t1{}) << std::endl;  // print 1
    std::cout << isSubsetOf(t1{}, t3{}) << std::endl;  // print 1
    std::cout << isSubsetOf(t3{}, t1{}) << std::endl;  // print 0
 }