Got it! Apparently when I copied all the headers I missed one that had a lock symbol on the bottom left corner. I guess the lock means it has to be copied individually; it can't be copied with the group so I had missed it? Does anyone know what this lock means? I'm surprised I did not get an error that says, "GUI.h missing".

I will try a simpler test program. Piecemeal growth is the way to steady progress.

String is written in #include "../../std_lib_facilities.h"

That header file was written by Stroustrup (the Creator of C++). Similarly, the other headers and cpp files are written by Stroustrup and I have tested them in another project. I am almost certain the problem is a linking problem. Perhaps this project is reading the source files in a different order than my first project so that the String class is being used before it has been declared or defined. How can I change the order in which the files are ?preprocessed? (not sure if that is the proper term)?

Here is the code that I wrote. I don't see the point in posting 12 seperate source files when i doubt anyone will parse them all. String is supposed to begin with a capital because Stroustrup wrote a class called String which checks strings for error whenever a string is invoked.

/*
 *  Taylor15_5.cpp
 *  TaylorGraph
 *
 *  Created by nathaniel on 9/19/12.
 *  Copyright 2012 __MyCompanyName__. All rights reserved.
 *
 *
 *	From Stroustrup's Programming: Principles and Practice
 *	Sectionn 15.5 Approximation, book example
 *
 *	The Taylor approximation gets worse with terms beyond a_13
 *	due to numerical roundoff error.
 *
 */

#include "Taylor15_5.h"
#include "../../std_lib_facilities.h"


#include "Simple_window.h"
#include "Graph.h"
#include "arc.h"
#include <stdio.h>
#include <math.h>
#include <iostream>

using namespace Graph_lib;


int fac(int n)	//factorial(n); n!
{
int r=1;
while (n>1){
r*=n;
--n;
}
return r;
}

double term(double x, int n){ return pow(x,n)/fac(n); }	//nth term of series

double expe(double x, int n)	//sum of n terms for x
{
double sum = 0;
	for (int i=0; i<n; ++i) sum+=term(x,i);
return sum;
}

int expN_number_of_terms=10;

double expN(double x)
{
return expe(x,expN_number_of_terms);
}

int main(){
	//Global constants
	const int xmax=600;
	const int ymax=400;
	
	const int x_orig=xmax/2;	//position of (0,0) is center of window
	const int y_orig=ymax/2;
	const Point orig(x_orig,y_orig);
	
	const int r_min=-10;		//range [-10:11)
	const int r_max=11;
	
	const int n_points = 400;	//number of points used in range
	
	const int x_scale = 30;		//scaling factors
	const int y_scale = 30;
	
	Simple_window win(Point(100,100),xmax,ymax,"Function graphing");
	
	for (int n = 0; n<50; ++n) {
		ostringstream ss;
		ss<<"exp approximation; n=="<< n;
		win.set_label(ss.str().c_str());
		expN_number_of_terms = n;
		//get next approximation:
		Function e(expN,r_min,r_max,orig,200,x_scale,y_scale);
		win.attach(e);
		win.wait_for_button();
		win.detach(e);
	}
}

I missed your post earlier. Yeah, I was using ^ instead of pow. No doubt that was my problem. Thanks!!