Question

Trying to compile the code below in Visual Studio 2010 gives me two errors about an unresolved external symbol. I’m pretty sure the issue is because I am using a function within a function but I don’t know specifically what I am doing incorrectly. The code looks ok to me.

#include <iostream>

using namespace std;

class fraction
{
  public:
    int gcd(int, int);
    void simplifyfraction();
  private:
    int integral, numerator, denominator;
};

int main(){
    return 0;
}

int gcd(int numerator, int denominator)
{
    if (denominator > numerator)
       return gcd(denominator, numerator);
    if (numerator == denominator)
       return numerator;
    if (numerator%denominator == 0)
       return denominator;
    return gcd(numerator, numerator-denominator);
}


void fraction::simplifyfraction()
{
    int mygcd;
    mygcd = gcd(numerator, denominator);
    numerator = numerator/mygcd;
    denominator = denominator/mygcd;
}

Answer 1

The definition of gcd() is not qualified with the class name and is treated as a free standing function, a different function from the member function gcd() declared in the fraction class.

As gcd() is invoked from fraction::simplifyfraction() the compiler searches for a function named gcd() and the first it finds is declared in the same scope as simplifyfraction(), namely fraction::gcd() which has no definition (it does not find the fully defined freestanding gcd() function). The linker subsequently complains that it has an unresolved symbol.

To correct change to:

int fraction::gcd(int numerator, int denominator)
{
}

---

Just to note this could also have been fixed by not having gcd() as a member function (it does not access any member variables of fraction, all variables are passed as arguments) or the invocation of gcd() in fraction::simplifyfraction() could be changed to:

mygcd = ::gcd(numerator, denominator);