Is There Anything Like a Templatized Case-Statement

Question

So I have this really ugly code:

template 
std::conditional_t

        

Answer 1

A type tag:

templatestruct tag{using type=T;};

void_t (coming in C++17 to a compiler near you):

templatestruct voider{using type=void;};
templateusing void_t=typename voider::type;

enable_first_t takes a pack of std::enable_if (note the lack of _t), and returns the first that passes the test. You can use tag to replace std::enable_if:

templatestruct has_type:std::false_type{};
templatestruct has_type>:std::true_type{};

namespace details {
  template
  struct enable_first {};
  template
  struct enable_first{} >, T0, Ts... >:enable_first {};
  template
  struct enable_first{} >, T0, Ts...>:T0 {};
}

templateusing enable_first_t=typename details::enable_first::type;

template
using result = enable_first_t<
  std::enable_if,
  std::enable_if,
  std::enable_if,
  tag // default
>;

this behaves a lot like a switch, but the statements are full boolean expressions.

live example