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

int main(int argc, char **argv) {

    int dieRoll = Dice::roll(6);
    std::cout<<dieRoll<<std::endl;

    std::cin.get();

    return 0;
}

#ifndef DieH
#define DieH

namespace Dice
{
    int roll(unsigned int dieSize);
}

#endif

#include <ctime>
#include <cstdlib>
#include "Dice.h"

namespace Dice
{
    int roll(unsigned int dieSize)
    {
    	if (dieSize == 0)
    	{
    		return 0;
    	}
    	srand((unsigned)time(0));
    	int random_int = 0;
    	random_int = rand()%dieSize+1;

    	return random_int;
    }
}

g++ -o program main.cpp Dice.cpp

Undefined symbols:
"Dice::roll(int)", referenced from:
  _main in ccYArhzP.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

g++ main.cpp -c

g++ dice.cpp -c

g++ main.o dice.o

nm main.o dice.o

main.o:
00000000 b .bss
00000000 d .ctors
00000000 d .data
00000000 r .eh_frame
00000000 t .text
00000098 t __GLOBAL__I_main
00000069 t __Z41__static_initialization_and_destruction_0ii
         U __ZN4Dice4rollEj
         U __ZNSi3getEv
         U __ZNSolsEPFRSoS_E
         U __ZNSolsEi
         U __ZNSt8ios_base4InitC1Ev
         U __ZNSt8ios_base4InitD1Ev
         U __ZSt3cin
         U __ZSt4cout
         U __ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
00000000 b __ZStL8__ioinit
         U ___gxx_personality_v0
         U ___main
00000055 t ___tcf_0
         U _atexit
00000000 T _main

dice.o:
00000000 b .bss
00000000 d .data
00000000 t .text
00000000 T __ZN4Dice4rollEj
         U _rand
         U _srand
         U _time

// Note, it is an int, not unsigned int
int roll(int dieSize)

main.o:
00000000 b .bss
00000000 d .ctors
00000000 d .data
00000000 r .eh_frame
00000000 t .text
00000098 t __GLOBAL__I_main
00000069 t __Z41__static_initialization_and_destruction_0ii
         U __ZN4Dice4rollEj
         U __ZNSi3getEv
         U __ZNSolsEPFRSoS_E
         U __ZNSolsEi
         U __ZNSt8ios_base4InitC1Ev
         U __ZNSt8ios_base4InitD1Ev
         U __ZSt3cin
         U __ZSt4cout
         U __ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
00000000 b __ZStL8__ioinit
         U ___gxx_personality_v0
         U ___main
00000055 t ___tcf_0
         U _atexit
00000000 T _main

dice.o:
00000000 b .bss
00000000 d .data
00000000 t .text
00000000 T __ZN4Dice4rollEi
         U _rand
         U _srand
         U _time

undefined reference to `Dice::roll(unsigned int)'