CS 330 Lecture 28 – Parametric Polymorphism
Agenda
- what ?s
- program this
- the cost of subtype polymorphism
- a Set class in C++
- a Set class in Java
- C++ templates vs. Java generics
Learning Intentions
- I understand that some methods of making code reusable lead to costs in performance and error detection.
- I can use templates to achieve parametric polymorphism in C++.
I can identify some of of the restrictions of Java’s implementation of parametric polymorphism.
Program This
You are writing a Set class. It manages a collection of unique objects. In pseudocode, write one of the following with your neighbor:
- A constructor that allocates unused space for the collection.
- A
Containsmethod that determines if an item already is in the set. - An
+=method that adds an item to the set, if it’s not already a member.
Code
Set.h
#ifndef SET_H
#define SET_H
#include <cassert>
template<typename T> class Set {
public:
Set() :
capacity(10),
nitems(0),
items(new T[capacity]) {
}
int GetSize() const {
return nitems;
}
const T& operator[](int i) const {
assert(i >= 0 && i < nitems);
return items[i];
}
bool Contains(const T& item) const {
for (int i = 0; i < nitems; ++i) {
if (items[i] == item) {
return true;
}
}
return false;
}
void operator+=(const T& item) {
if (Contains(item)) {
return;
}
// Expand as necessary
if (nitems == capacity) {
capacity *= 2;
T *bigger = new T[capacity];
for (int i = 0; i < nitems; ++i) {
bigger[i] = items[i];
}
delete[] items;
items = bigger;
}
// Append
items[nitems] = item;
++nitems;
}
private:
int capacity;
int nitems;
T *items;
};
#endif
main.cpp
#include <iostream>
#include "Set.h"
class Point2D {
public:
Point2D() : x(0), y(0) {
}
Point2D(int x, int y) : x(x), y(y) {
}
bool operator==(const Point2D& other) {
return x == other.x && y == other.y;
}
private:
int x, y;
};
int main(int argc, char **argv) {
Set<int> ns;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 12;
ns += 21;
/* ns += std::exception(); */
std::cout << "ns.GetSize(): " << ns.GetSize() << std::endl;
Set<Point2D> points;
points += Point2D(5, 10);
points += Point2D(1, 2);
points += Point2D(3, 4);
std::cout << "points.GetSize(): " << points.GetSize() << std::endl;
return 0;
}
Haiku
First it was New York
Now you can ♥ anything
‘Cuz they are <T>-shirts
Now you can ♥ anything
‘Cuz they are <T>-shirts