Coderbyte is an online coding challenge site (I found it just 2 minutes ago).

The first C++ challenge you are greeted with has a C++ skeleton you need to modify:

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

int FirstFactorial(int num) {

  // Code goes here
  return num;

}

int main() {

  // Keep this function call here
  cout << FirstFactorial(gets(stdin));
  return 0;

}

If you are little familiar with C++ the first thing* that pops in your eyes is:

int FirstFactorial(int num);
cout << FirstFactorial(gets(stdin));

So, ok, the code calls gets which is deprecated since C++11 and removed since C++14 which is bad in itself.

But then I realize: gets is of type char*(char*). So it shouldn’t accept a FILE* parameter and the result shouldn’t be usable in the place of an int parameter, but … not only it compiles without any warnings or errors, but it runs and actually passes the correct input value to FirstFactorial.

Outside of this particular site, the code doesn’t compile (as expected), so what is going on here?


I’m the founder of Coderbyte and also the guy who created this gets(stdin) hack.

The comments on this post are correct that it is a form of find-and-replace, so let me explain why I did this really quickly.

Back in the day when I first created the site (around 2012), it only supported JavaScript. There was no way to “read in input” in JavaScript running in the browser, and so there would be a function foo(input) and I used the readline() function from Node.js to call it like foo(readline()). Except I was a kid and didn’t know better, so I literally just replaced readline() with the input at run-time. So foo(readline()) became foo(2) or foo("hello") which worked fine for JavaScript.

Around 2013/2014 I added more languages and used third-party service to evaluate code online, but it was very difficult to do stdin/stdout with the services I was using, so I stuck with the same silly find-and-replace for languages like Python, Ruby, and eventually C++, C#, etc.

Fast forward to today, I run the code in my own containers, but never updated the way stdin/stdout works because people have gotten used to the weird hack (some people have even posted in forums explaining how to get around it).

I know it is not best practice and it isn’t helpful for someone learning a new language to see hacks like this, but the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem. One common complaint about coding challenge sites years ago was that new programmers would spend a lot of time just figuring out how to read from stdin or read lines from a file, so I wanted new coders to avoid this problem on Coderbyte.

I’ll be updating the entire editor page soon along with the default code and stdin reading for languages. Hopefully then C++ programmers will enjoy using Coderbyte more 🙂