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.

*like other built-in functions* which ones are you referring to? — Borgleader, Jun 17 '15 at 17:39
@JerryGoyal all those are standard library functions (as in, they are required to be there by the C++ standard), __gcd is gcc specific and therefore not portable. [Related question](http://stackoverflow.com/questions/13443463/built-in-functions-in-c) — Borgleader, Jun 17 '15 at 17:41

score 16 · Accepted Answer · answered Jun 17 '15 at 17:44

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.

score 5 · Answer 2 · edited May 09 '20 at 08:39

5

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);
}

edited May 09 '20 at 08:39

Brett Hale

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answered May 09 '20 at 08:07

Kuji

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

2 Answers2