How to make a variadic is_same? - c++

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<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 1

Nice and concise with C++17:
template <class T, class... Ts>
struct is_any : std::disjunction<std::is_same<T, Ts>...> {};
And the dual:
template <class T, class... Ts>
struct are_same : std::conjunction<std::is_same<T, Ts>...> {};
A variation that uses fold expressions:
template <class T, class... Ts>
struct is_any : std::bool_constant<(std::is_same_v<T, Ts> || ...)> {};
template <class T, class... Ts>
struct are_same : std::bool_constant<(std::is_same_v<T, Ts> && ...)> {};

Answer 2

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

Answer 3

In C++17 you have an even nicer solution, using template variables and fold expressions:
template<class T, class... Rest>
inline constexpr bool are_all_same = (std::is_same_v<T, Rest> && ...);
And the usage is also simpler than all other examples:
are_all_same<T, A, B, C>
No ::value, no parentheses!

Answer 4

Something like this. First, a small metaprogramming library, because it adds like 2 lines to do it generically:
template<template<typename,typename>class checker, typename... Ts>
struct is_any_to_first : std::false_type {};
template<template<typename,typename>class checker, typename T0, typename T1, typename... Ts>
struct is_any_to_first<checker, T0, T1, Ts...> :
std::integral_constant< bool, checker<T0, T1>::value || is_any_to_first<checker, T0, Ts...>::value>
{};
Then a 2 line implementation of is_any_same_to_first:
template<typename... Ts>
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:
template<template<typename,typename>class checker, typename... Ts>
struct is_all : std::true_type {};
template<template<typename,typename>class checker, typename T0, typename T1, typename... Ts>
struct is_all<checker, T0, T1, Ts...> :
std::integral_constant< bool, checker<T0, T1>::value && is_all<checker, T0, Ts...>::value>
{};
template<typename... Ts>
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<A,B,C> is true iff all three are the same type!

Answer 5

Using the relaxed C++14 constexpr functions, these kinds of things are much easier to code, and probably much faster to compile as well, so you could write:
template <class T, class ... Candidates>
constexpr bool is_all_same() {
bool pairs[] = {std::is_same<T,Candidates>::value...};
for(bool p: pairs) if(!p) return false;
return true;
}
template <class T, class ... Candidates>
constexpr bool is_any_same() {
bool pairs[] = {std::is_same<T,Candidates>::value...};
for(bool p: pairs) if(p) return true;
return false;
}
This is enabled by the fact that in C++14 constexpr functions can have for loops.

Answer 6

The most generic version that works :
since C++11
without dependencies (no #include <type_traits> needed)
only one name is_same works for n-types (no is_same_all needed)
template <typename First, typename Second, typename ... Next>
struct is_same {
template <typename A, typename B>
struct is_same_min {
enum { value = false };
};
template <typename A>
struct is_same_min<A,A> {
enum { value = true };
};
template <typename X, typename Y>
constexpr static bool check() {
return is_same_min<X,Y>::value;
};
template <typename X, typename Y, typename Z, typename ... K>
constexpr static bool check() {
return is_same_min<X,Y>::value and check<Y, Z, K...>();
};
enum { value = check<First, Second, Next...>() };
};
Just use is_same<T1,T2,T3...>::value