Function Referenced from Function: Symbol not Found

[TechnoCat]

I'm getting this error from Xcode:

"digitsIn(int)", referenced from:
      binaryToDecimal(int)in Binary Conversion.o
      binaryToDecimal(int)in Binary Conversion.o
      binaryToDecimal(int)in Binary Conversion.o
      binaryToDecimal(int)in Binary Conversion.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

digitsIn (int) is a function I wrote in another application (given an integer, it finds the number of digits in that integer) . I've included it by including the header file from that application

// header.h from this project:
#include "[path to other project]/header.h"
#include <cmath>
#include <iostream>
using namespace std ;

int decimalToBinary (int const decimalNum) ;
int binaryToDecimal (int const binaryNum) ;

// header.h from other project:
#include <iostream>
using namespace std ;

int digitsIn (int const num) ;

// from main.cpp (this project)
int binaryToDecimal (int const binaryNum) {
	int h = binaryNum ;
	int binaryNum_array [digitsIn (binaryNum) ] ;
	int decimalNum ;
	
	for (int i = 0 ; i <= digitsIn (binaryNum) ; i ++ ) {
		binaryNum_array [i] = h % 10 ;
		h /= 10 ;
		}
	
	for (int i = digitsIn (binaryNum) ; i >= 0 ; i -- ) {
		decimalNum += pow (d2, digitsIn (binaryNum) - i) ;
		}
	
	return decimalNum ;
	}

[hamsterman]

The implementation of digitsIn is not in any of the files that are compiled (cpp files in your current project), so compiler doesn't find it.

[TechnoCat]

So I just add a reference to that .cpp file, right?

[hamsterman]

What do you mean be reference?
You could add that file to your project, or you could simply copy the function into one of your current files.

[anonymous23323124]

So I just add a reference to that .cpp file, right?

To clarify, you don't mean by this to #include the cpp file, right? This is not a good idea...