Me myself and I Me myself and I - 3 months ago 25
C++ Question

How to make a variadic is_same?

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<T, A, B, C>::value; // true if T is one of A, B or C


And if
T
is equal to any one of those types, its static
value
member will be
true
, otherwise
false
. How can I do this?

Answer

Use template recursion:

template<typename T, typename... Rest>
struct is_any : std::false_type {};

template<typename T, typename First>
struct is_any<T, First> : std::is_same<T, First> {};

template<typename T, typename First, typename... Rest>
struct is_any<T, First, Rest...>
    : std::integral_constant<bool, std::is_same<T, First>::value || is_any<T, Rest...>::value>
{};

static_assert(is_any<int, char, double, int>::value, "error 1");   // OK
static_assert(is_any<int, char, double, short>::value, "error 2"); // error