Inbuilt __gcd(A,B) function in C++

Question

recently I get to know about a special function in c++ : __gcd(A,B). this will return the greatest common divisor of A and B.

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

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

is there any special reason for starting function definition with 2 underscores?

It could be as simple as gcd(A,B) like other STL functions.

Answer 1

In C++17 there are standard library functions for GCD and LCM.

#include <iostream>
#include <numeric>

int main ()
{
    int a, b;
    std::cin >> a >> b;
    std::cout << std::gcd(a,b) << '\n';
    return (0);
}