Check if parameter pack contains a type

Question

I was wondering if C++0x provides any built-in capabilities to check if a parameter pack of a variadic template contains a specific type. Today, boost:::mpl::contains can be

Answer 1

If you want to avoid manual type recursion, std::common_type appears to me to be the only utility in the STL which is a variadic template, and hence the only one which could potentially encapsulate recursion.

Solution 1

std::common_type finds the least-derived type in a set of types. If we identify numbers with types, specifically high numbers with less-derived types, it finds the greatest number in a set. Then, we have to map equality to the key type onto a level of derivation.

using namespace std;

struct base_one { enum { value = 1 }; };
struct derived_zero : base_one { enum { value = 0 }; };

template< typename A, typename B >
struct type_equal {
 typedef derived_zero type;
};

template< typename A >
struct type_equal< A, A > {
 typedef base_one type;
};

template< typename Key, typename ... Types >
struct pack_any {
 enum { value =
     common_type< typename type_equal< Key, Types >::type ... >::type::value };
};

Solution 2

We can hack common_type a little more. The standard says

A program may specialize this trait if at least one template parameter in the specialization is a user-defined type.

and describes exactly what is inside it: a recursive partial specialization case, a case which applies a binary operator, and a terminal case. Essentially, it's a generic fold function, and you can add whatever binary operation you please. Here I used addition because it's more informative than OR. Note that is_same returns an integral_constant.

template< typename Addend >
struct type_sum { // need to define a dummy type to turn common_type into a sum
    typedef Addend type;
};

namespace std { // allowed to specialize this particular template
template< typename LHS, typename RHS >
struct common_type< type_sum< LHS >, type_sum< RHS > > {
    typedef type_sum< integral_constant< int,
     LHS::type::value + RHS::type::value > > type; // <= addition here
};
}

template< typename Key, typename ... Types >
struct pack_count : integral_constant< int,
 common_type< type_sum< is_same< Key, Types > > ... >::type::type::value > {};