Introduction to Programming Languages/Parametric Polymorphism

Parametric polymorphism

Parametric polymorphism occurs when a routine, type or class definition is parameterized by one or more types. It allows the actual parameter type to be selected by the user. This way, it is possible to define types or functions that are generics, which can be expressed by using type variables for the parameter type. The code below shows the use of a template, which is a way of implementing parametric polymorphism in C++.

#include <iostream>

template <class T>
T GetMax (T a, T b) {
  T result;
  result = (a > b) ? a : b;
  return (result);
}

int main() {
  int i = 5, j = 6, k;
  long l = 10, m = 5, n;
  k = GetMax<int>(i, j);       // type parameter: int
  n = GetMax<long>(l, m);      // type parameter: long
  std::cout << k << std::endl;
  std::cout << n << std::endl;
  return 0;
}

The source code above defines a polymorphic function called GetMax (lines 3 to 8). The type variable T defined in the scope of GetMax is a kind of generics, which will be substituted at the function call. The function takes two parameters (a and b) and returns a value of type T. The runtime values of a and b are compared and the bigest is returned by the function. The main function shows two calls for the GetMax function. At line 13 the function call uses the type int whereas at line 14 it uses the type long. The content of the arguments in a GetMax function call is compared using the ">" operator. For using this function, it is necessary that a variable type has this operator implemented. The following code shows an implementation of the overloading operator ">" in the class MyInt, so that it is possible to apply GetMax on it. The main function shows its use.

#include <iostream>
 
class MyInt {
  friend std::ostream & operator<<(std::ostream& os, const MyInt& m) {
    os << m.data;
  }
  friend bool operator >(MyInt& mi1, MyInt& mi2) {
    return mi1.data > mi2.data;
  }
  public:
    MyInt(int i) : data(i) {}
  private:
    const int data;
};

template <class T>
T GetMax (T a, T b) {
  return (a > b) ? a : b;
}

int main () {
  MyInt m1(50), m2(56);
  MyInt mi = GetMax<MyInt>(m1, m2);
  std::cout << mi << std::endl;
  return 0;
}