"Undefined symbols for architecture x86_64: " Compiler Error

Question

I am writing a HugeInteger program in c++ and I am in need of some assistance. When I compile my code I get Undefined symbols for architecture x86_64: back through the compiler along with some jargon that I don't understand.

Here is my c++ source code:

#include "HugeInteger.h"
#include <iostream>
#include <vector>
#include <string>
using std::vector;
using std::string;

HugeInteger::HugeInteger(){
  vector<int> digit;
  digit[0]=0;
}

HugeInteger::HugeInteger(string largeNumber){
  string bigNumber;
  largeNumber= bigNumber;
}

Here is my header file code:

#pragma once
#include <string>
class HugeInteger

{

private:

public:
   HugeInteger();
   HugeInteger(std::string bigNumber );



};

Any help would be great,