How to make a variadic is_same?

Question

How can I make a class template that returns whether any of its variadic types are equal to the first type. I want to be able to do this:

is_same

        

Answer 1

Something like this. First, a small metaprogramming library, because it adds like 2 lines to do it generically:

templateclass checker, typename... Ts>
struct is_any_to_first : std::false_type {};

templateclass checker, typename T0, typename T1, typename... Ts>
struct is_any_to_first :
  std::integral_constant< bool, checker::value || is_any_to_first::value>
{};

Then a 2 line implementation of is_any_same_to_first:

template
using is_any_same_to_first = is_any_to_first< std::is_same, Ts... >;

And for completeness, the original is_all, which may also prove useful:

templateclass checker, typename... Ts>
struct is_all : std::true_type {};

templateclass checker, typename T0, typename T1, typename... Ts>
struct is_all :
  std::integral_constant< bool, checker::value && is_all::value>
{};

template
using is_all_same = is_all< std::is_same, Ts... >;

Live example of the is_all_same.

Note that calling is_any_same_to_first anything less explicit is asking for trouble. 2/3 people who tried to answer this question, including me, assumed that is_same is true iff all three are the same type!