Compiling a C++ Program with Headers (Newbie)

Question

I am learning C++ from Edward Schneinerman's C++ for Mathematicians. I am working on the greatest common divisor section in chapter 2. I have made three files:

gcd.h

#ifndef GCD_H
#define GCD_H

long gcd(long a, long b);

#endif

gcd.cc

#include "gcd.h"
#include<iostream>

using namespace std;

long gcd(long a, long b) {
    if( a == 0 && b == 0 ) {
        cerr << "WARNING: gcd(0,0) not defined" << endl;
        return 0;
    }

    if( a < 0 ) a = -a;
    if( b < 0 ) b = -b;

    if( b == 0 ) retrun a;
    if( a == 0 ) return b;

    long c = a % b;

    return gcd(b,c);
}

and gcdtest.cc

#include "gcd.h"
#include <iostream>

using namespace std;

int main() {
    long a,b;

    cout << "Enter the first number > ";
    cin >> a;
    cout << "Enter the second number > ";
    cin >> b;

    long d = gcd(a,b);

    cout << "The gcd of " << a << " and " << b << " is " << d << endl;

    return 0;
}

All of these files reside in the same directory. When I attempt to compile gcdtest.cc

$ g++ -Wall gcdtest.cc -o gcdtest.exe

I receive the following error:

$ g++ -Wall gcdtest.cc -o gcdtest.exe
Undefined symbols for architecture x86_64:
      "gcd(long, long)", referenced from:
          _main in ccgbCyQO.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

I am brand new to C++ and havent fully grokked compiling and linking. What am I doing wrong?

Answer 1

You should attempt to compile both C files:

$ g++ -Wall gcdtest.cc gcd.cc -o gcdtest.exe

Otherwise you have 'gcd' declared (in your header file), but compiler is missing its implementation.

Answer 2

Compile your program either this way:

g++ -Wall gcd.cc gcdtest.cc -o gcdtest.exe

Or this way:

g++ -Wall -c gcdtest.cc
g++ -Wall -c gcd.cc
g++ -Wall gcd.o gcdtest.o -o gcdtest.exe

Answer 3

You have to compile gcd.cc first.

$g++ -c gcd.cc -o gcd.o
$g++ gcdtest.cc gcd.o -o gcdtest.exe