#include <iostream>
using namespace std;

// file: mondias.cc
// using user-defined functions
// functions calling functions

int main()
{
   Diamond2();

   return 0;
}

void Diamond2()
{
   Diamond();
   Diamond();
}

void Diamond()
{
	Dia();
	Mond();
}

void Mond()
{
	cout << " ***** " << endl;
	cout << "  ***  " << endl;
	cout << "   *   " << endl;
}

void Dia()
{
	cout << "   *   " << endl;
	cout << "  ***  " << endl;
	cout << " ***** " << endl;
	cout << "*******" << endl;
}

Will not compile:

prompt> make mondias
g++ -I/usr/project/courses/cps008/lib -g mondias.cc -o mondias -lm /usr/project/courses/cps008/lib/libtapestry.a -lg++
mondias.cc: In function `int main()':
mondias.cc:9: warning: implicit declaration of function `int Diamond2(...)'
mondias.cc: In function `void Diamond2()':
mondias.cc:16: warning: implicit declaration of function `int Diamond(...)'
mondias.cc: In function `void Diamond()':
mondias.cc:22: warning: implicit declaration of function `int Dia(...)'
mondias.cc:23: warning: implicit declaration of function `int Mond(...)'
Undefined                       first referenced
 symbol                             in file
Dia                                 /tmp/cca003NS1.o
Diamond                             /tmp/cca003NS1.o
Mond                                /tmp/cca003NS1.o
Diamond2                            /tmp/cca003NS1.o
ld: fatal: Symbol referencing errors. No output written to mondias
*** Error code 1
make: Fatal error: Command failed for target `mondias'