Chap. 6 Function Templates
Can be considered, whenever you write several identical functions that perform the same operation on different data types. (cf. Function Overloading)

A function that returns the absolute value of a integer number 
 
int abs(int n)
{
   return (n < 0) ? -n : n;
}
A function that returns the absolute value of a float number 
 
float abs(float n)
{
   return (n < 0) ? -n : n;
}

...

Using a Function Template 
 
// template used for absolute value function
#include <iostream.h>

template <class T>             // function template
T abs(T n)
{
   return (n < 0) ? -n : n;
}

void main()
{
   int int1 = 5;
   int int2 = -6;
   long lon1 = 70000L;
   long lon2 = -80000;
   double dub1 = -9e-2;
   double dub2 = -10.15;
   
   cout << abs(int1) << endl;
   cout << abs(int2) << endl;
   cout << abs(lon1) << endl;
   cout << abs(lon2) << endl;
   cout << abs(dub1) << endl;
   cout << abs(dub2) << endl;
}