Instantiation

Since we may potentially have an infinite number of types and classes, the concept of function templates not only saves space in the source code file but also makes code easier to read and maintain. However, compared to writing separate functions or classes for the different data types that are used in our applications, it does not produce smaller object code. For instance, consider a program using a float and int version of app_max():

cout << app_max<int>(3,5) << endl;
cout << app_max<float>(3.0f,5.0f) << endl;

The compiler will generate two new functions in the object file, as follows:

int app_max<int> ( int a, int b) {
  return (a>b?a:b);
}

float app_max<float> (float a, float b) {
  return (a>b?a:b);
}

This process of creating a new definition of a function from a function template declaration is called template instantiation. During this instantiation process, the compiler determines the template arguments and generates actual functional code on demand for your application. Typically, there are three forms: explicit instantiations, implicit instantiations, and template deductions. In the next sections, let's discuss each form.