Check if a type is passed in variadic template parameter pack

问题

I've heard somewhere, that using new C++1z syntax, it is really easy to check if a type is passed in variadic template parameter pack - apparently you can do this with code that is near one-line long. Is this true? What are those relevant features? (I tried looking through fold expressions but I can't see how to use them in that problem...)

Here's how I solved the problem in C++11 for reference:

#include <type_traits>


template<typename T, typename ...Ts>
struct contains;

template<typename T>
struct contains<T> {
    static constexpr bool value = false;
};

template<typename T1, typename T2, typename ...Ts>
struct contains<T1, T2, Ts...> {
    static constexpr bool value = std::is_same<T1, T2>::value ? true : contains<T1, Ts...>::value;
};

回答1:

You're looking for std::disjunction. It's specified in N4564 [meta.logical].

#include <type_traits>

template<typename T, typename... Ts>
constexpr bool contains()
{ return std::disjunction_v<std::is_same<T, Ts>...>; }

static_assert(    contains<int,      bool, char, int, long>());
static_assert(    contains<bool,     bool, char, int, long>());
static_assert(    contains<long,     bool, char, int, long>());
static_assert(not contains<unsigned, bool, char, int, long>());

Live demo

Or, adapted to a struct

template<typename T, typename... Ts>
struct contains : std::disjunction<std::is_same<T, Ts>...>
{};

Or, using fold expressions

template<typename T, typename... Ts>
struct contains : std::bool_constant<(std::is_same<T, Ts>{} || ...)>
{};

Live demo

回答2:

If you are bound to C++11 you cannot use std::disjunction nor fold-expressions. However, it is rather straightforward to roll your own any_is_same:

template<typename same, typename first,typename...more> 
struct any_is_same {
    static const bool value = std::is_same<same,first>::value || 
                              any_is_same<first,more...>::value; 
};

template<typename same,typename first> 
struct any_is_same<same,first> : std::is_same<same,first> {};


int main(){
    std::cout << any_is_same<int, int,double,float>::value << "\n";
    std::cout << any_is_same<std::string, int,double,float>::value << "\n";
}

Live Demo

来源：https://stackoverflow.com/questions/34099597/check-if-a-type-is-passed-in-variadic-template-parameter-pack

标签

c++

templates

typetraits

c++17