What's the logic behind the order the elements are passed to a comparison function in std::sort?

Question

I'm practicing lambdas:

 int main()
 {
    std::vector<int> v {1,2,3,4};
    int count = 0;
    sort(v.begin(), v.end(), [](const int& a, const int& b) -> bool
        {
        return a > b;
        });
  }

This is just code from GeeksForGeeks to sort in descending order, nothing special. I added some print statements (but took them out for this post) to see what was going on inside the lambda. They print the entire vector, and the a and b values:

1 2 3 4  
a=2 b=1

2 1 3 4  
a=3 b=2

3 2 1 4  
a=4 b=3

4 3 2 1 <- final

So my more detailed question is: What's the logic behind the order the vector elements are being passed into the a and b parameters?

Is b permanently at index 0 while a is iterating? And if so, isn't it a bit odd that the second param passed to the lambda stays at the first element? Is it compiler-specific? Thanks!

Answer 1

By passing a predicate to std::sort(), you are specifying your sorting criterion. The predicate must return true if the first parameter (i.e., a) precedes the second one (i.e., b), for the sorting criterion you are specifying.

Therefore, for your predicate:

return a > b;

If a is greater than b, then a will precede b.

---

So my more detailed question is: What's the logic behind the order the vector elements are being passed into the a and b parameters?

a and b are just pairs of elements of the elements you are passing to std::sort(). The "logic" will depend on the underlying algorithm that std::sort() implements. The pairs may also differ for calls with identical input due to randomization.