What does std::includes actually do?

Question

From the standard, std::includes:

Returns: true if [first2, last2) is empty or if every element in the range

Answer 1

I believe you're trying to check if a includes b in your example, a doesn't include b but b does include a. If you swap b and a it will return true, since a is included in b.

I hope I'm not missing something obvious.

#include 
#include 
#include 

int main() {
    std::vector a({1});
    std::vector b({1,1,1});

    // Outputs 'true'
    std::cout << std::boolalpha
        << std::includes(b.begin(), b.end(), a.begin(), a.end()) << '\n';
}

What I've understood by playing around with algorithm is, when you type includes(R2, R1) it checks if R2 owns R1 as a subgroup, if yes returns true if not returns false. Also if it's not ordered throws an error: sequence not ordered.