Match ranges against individual items

How about a function that tells whether some range contains at least one of the values we provide as variadic parameters:

template <typename R, typename ... Ts>
auto matches(const R& range, Ts ... ts)
{
 return (std::count(std::begin(range), std::end(range), ts) + ...);
}

The helper function uses the std::count function from the STL. This function takes three parameters: the first two parameters are the begin and end iterators of some iterable range, and as the third parameter, it takes a value which will be compared to all the items of the range. The std::count method then returns the number of all the elements within the range that are equal to the third parameter.

In our fold expression, we always feed the begin and end iterators of the same parameter range into the std::count function. However, as the third parameter, each time we put one other parameter from the parameter pack into it. In the end, the function sums up all the results and returns it to the caller.

We can use it like this:

std::vector<int> v {1, 2, 3, 4, 5};

matches(v, 2, 5); // returns 2
matches(v, 100, 200); // returns 0
matches("abcdefg", 'x', 'y', 'z'); // returns 0
matches("abcdefg", 'a', 'd', 'f'); // returns 3

As we can see, the matches helper function is quite versatile--it can be called on vectors or even on strings directly. It would also work on initializer lists, on instances of std::list, std::array, std::set, and so on!