stable_sort on a vector of pairs by first element in pair in increasing order without comparator function in C++

Question

#include  
using namespace std; 

int main() 
{ 

    vector>v;
    v.push_back(ma         


        

Answer 1

The comparator used uses both values in the pairs when doing the comparison.

It compares lhs and rhs lexicographically by operator< (operator<=> since C++20), that is, compares the first elements and only if they are equivalent, compares the second elements.

If I write the comparator function, then it is working properly

That's because you've probably implemented it like this:

[](const auto& lhs, const auto& rhs) {
    return lhs.first < rhs.first;
}

The default comparator does something equivalent to this:

[](const auto& lhs, const auto& rhs) {
    return lhs.first == rhs.first ? lhs.second < rhs.second : lhs.first < rhs.first;
}