16.1. Defining a Template

Imagine that we want to write a function to compare two values and indicate whether the first is less than, equal to, or greater than the second. In practice, we’d want to define several such functions, each of which will compare values of a given type. Our first attempt might be to define several overloaded functions:

// returns 0 if the values are equal, -1 if v1 is smaller, 1 if v2 is smaller
int compare(const string &v1, const string &v2)
{
    if (v1 < v2) return -1;
    if (v2 < v1) return 1;
    return 0;
}
int compare(const double &v1, const double &v2)
{
    if (v1 < v2) return -1;
    if (v2 < v1) return 1;
    return 0;
}

These functions are nearly identical: The only difference between them is the type of their parameters. The function body is the same in each function.

Having to repeat the body of the function for each type that we compare is tedious and error-prone. More importantly, we need to know when we write the program all the types that we might ever want to compare. This strategy cannot work if we want to be able to use the function on types that our users might supply.