Inbuilt gcd A B function in C

Question

I recently learned about a c++ special function called __gcd (A,B). 

The greatest common divisor of A and B will be returned.

#include<iostream>
#include<algorithm>
using namespace std;

main()
{
cout<<__gcd(10,40); //op: 10
}

Is there a reason why the function definition begins with two underscores?

Like other STL functions, it might be as easy as gcd(A,B).

Answer 1

Names starting with two underscores are reserved for the implementation, which means that you are not allowed to define such names in your code, and there are no standard guarantees what those names mean if they do exist. However, a vendor might choose to document some such names, in which case you can use them with the product for which the vendor documents them.