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

No, you have to use (partial) specialization with variadic templates to do compile-time computations like this:

#include 

template < typename Tp, typename... List >
struct contains : std::true_type {};

template < typename Tp, typename Head, typename... Rest >
struct contains
: std::conditional< std::is_same::value,
    std::true_type,
    contains
>::type {};

template < typename Tp >
struct contains : std::false_type {};

There is only one other intrinsic operation for variadic templates and that is the special form of the sizeof operator which computes the length of the parameter list e.g.:

template < typename... Types >
struct typelist_len
{
   const static size_t value = sizeof...(Types);
};

Where are you getting "it has serious compilation-time overhead" with boost mpl from? I hope you are not just making assumptions here. Boost mpl uses techniques such as lazy template instantiation to try and reduce compile-times instead of exploding like naive template meta-programming does.