为什么 std::set_difference 不删除重复项？

Question

cppreference states that std::set_difference :

Copies the elements from the sorted range [first1, last1) which are not found in the sorted range [first2, last2) to the range beginning at d_first.

With that said, I would expect the following MWE to output 2 4 , because only 2 and 4 are not in the second set.

#include <algorithm>
#include <set>
#include <vector>
#include <iostream>

int main() {
    std::vector<int> A{1, 1, 2, 2, 2, 3, 3, 4};
    std::set<int> B{1, 3};
    std::set<int> difference;
    std::set_difference(
        A.cbegin(), 
        A.cend(), 
        B.cbegin(),
        B.cend(), 
        std::inserter(difference, difference.end())
    );

    std::for_each(difference.cbegin(), difference.cend(), [](int n) {
        std::cout << n << " ";
    });
    std::cout << std::endl;
}

However, it appears that this outputs 1 2 3 4 , and I'm not fully sure why. It does what I expect if I change A to only contain 1 2 3 4 Can someone help me understand this?

Answer 1

I'm not really sure there's a good reason it has to be defined to work that way, but it definitely is how it's defined. The standard specifically requires this behavior. Here's the wording as of N4835 (§[set.difference]/6):

Remarks: If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the last max(m − n, 0) elements from [first1, last1) is copied to the output range, in order.