Program works when executed in C++ Tutor, but not anywhere else

Question

I'm writing a rather simple program following Euclidean algorithm (we have to write it using recursion). It works really well when executed in C++ Tutor but when I compile the program in the terminal, it already gives me:

warning: control may reach end of non-void function [-Wreturn-type]

When I try to execute it in the terminal, it throws:

runtime error: execution reached the end of a value-returning function without returning a value

(But I am returning a value?)

Why does it work with c++ tutor but not in the Linux terminal (using clang compiler)?

I tried to use a bunch of extra variables in the function to make the process clearer to me, but I still don't get why it thinks that there would be a case where I would not return a value.

#include <iostream>

using namespace std;

int ggt(int a, int b){

    int rest{0};
    int zaehler{0};
    int divisor{0};

    if(a>=b){
        zaehler=a;
        divisor=b;

        if(a%b==0){
            return b;   
        }
        else{
            rest=a%b;
            divisor=rest;
            zaehler=b;

            ggt(zaehler, divisor);
        }
    }
    else{
        zaehler=b;
        divisor=a;

        if(b%a==0){
            return a;   
        }
        else{
            rest=b%a;
            divisor=rest;
            zaehler=a;

            ::durchlaeufe--;
            ggt(zaehler, divisor);

        }
    }
}

int main(){

int a{40}, b{12};

    cout << "Bitte Zaehler eingeben: ";
    cin >> a;
    cout << "\n";

    cout << "Bitte Nenner eingeben: ";
    cin >> b;
    cout << "\n";

    if(ggt(a, b)==0){
        cout << "ERROR\n";
    }
    else   {
    cout << "Der groesste gemeinsame Teiler ist: " << ggt(a, b) << "\n";
    }

    return 0;
}

In this example, with a=40 and b=12, the result should be 4. And that's exactly what C++ tutor says...

In most branches `ggt` doesn't return anything. A function with a type other than `void` (and that isn't `main`) is required to `return` before control reaches the end of the function body. Edit : You probably want to change both `ggt(zaehler, divisor);` to `return ggt(zaehler, divisor);`. — François Andrieux, Apr 15 '19 at 19:17
Possible duplicate of [Error: control may reach end of non-void function in C](https://stackoverflow.com/questions/22410538/error-control-may-reach-end-of-non-void-function-in-c) — Kevin Chen, Apr 15 '19 at 19:19
Omg thats it thank you, I dont know why I did not see that.. — David würflinger, Apr 15 '19 at 19:20
For what it's worth, the compilers that online C++ tutorials use sometimes do really weird things. You'd be better off with a compiler where you can read every warning. My guess would be that C++ Tutor is suppressing or hiding warnings from you and that your code relies on Undefined Behavior. [Here](https://stackoverflow.com/questions/55269252/what-is-going-on-with-getsstdin-on-the-site-coderbyte) is another example of a really weird compiler from a C++ tutorial — alter_igel, Apr 15 '19 at 19:22
A good time to enable ALL compiler warnings! And not any should be silently ignored! — Klaus, Apr 15 '19 at 19:24
If the compiler has the option (like `-Werror`) to make warnings errors, take advantage of it. This forces you to deal with them before having to pull out the debugger to find out what went wrong at runtime. — user4581301, Apr 15 '19 at 19:53

skeller · Accepted Answer · 2019-04-15T20:06:54.783

the actual answer (missing return at recursion call) was already given.

i'd like to add a simpler version for you to compare and maybe learn something :)

int ggt(int a, int b)
{
    if (a < b)
    {
        std::swap(a, b);
    }

    if (a%b == 0)
    {
         return b;
    }

    return ggt(b, a%b);
 }

short explanation:

the "swap" when a < b ensures b contains the smaller value.
the "rest" (a%b) is calculated directly when calling the recursion (this avoids storing intermediate values)
as you see the control flow is simpler, so it is easier to reason about each step of the execution.

Program works when executed in C++ Tutor, but not anywhere else

1 Answers1