Week 03 / 14 Object-Oriented Programming

Lab 3 - From requirements to classes and encapsulation

We turn a short list of requirements into a Product class. First we design it on paper, then we hide its state and write methods that never let it become invalid.

Plan (90 min)

Requirements

A shop keeps products. Each product has an id, a name, a price and a stock (how many pieces are left). The shop can change the price, sell some pieces, restock and ask for the value of the stock (price times stock).

Design first, no code

Starting program

This is how the data looks without a class. Read it and predict the last line before you run it.

#include <iostream>
#include <string>
using namespace std;

struct Product {
    int id;
    string name;
    double price;
    int stock;
};

double value(Product p) {
    return p.price * p.stock;
}

int main() {
    Product p;
    p.id = 1;
    p.name = "Pen";
    p.price = 2.5;
    p.stock = 10;

    p.stock = p.stock - 15;   // sold 15 pens we did not have
    p.price = -1;             // typo while changing the price
    p.id = 7;                 // someone "fixed" the id

    cout << p.id << " " << p.name << " " << p.stock << " " << value(p) << endl;
}

It prints 7 Pen -5 5. The stock value is positive and looks reasonable, because two wrong numbers multiplied give a plausible one. Every line in main compiled. Nothing in the program knows the rules of a shop.

Mandatory questions

1. What is the object’s state?
2. What behaviour belongs to the object?
3. What states should be impossible?
4. What should be private? What should be public?
5. Which properties should not have setters?
6. What is this class responsible for?

Predict, then check:

#include <iostream>
using namespace std;
class Product {
    double price = 0;
    int stock = 0;
public:
    void init(double p, int s) { price = p; stock = s; }
    bool sell(int qty) {
        if (qty <= 0 || qty > stock) return false;
        stock -= qty;
        return true;
    }
    int getStock() const { return stock; }
    double getValue() const { return price * stock; }
};
int main() {
    Product p;
    p.init(2.5, 10);
    p.sell(3);
    p.sell(20);
    cout << p.getStock() << " " << p.getValue();
}

Minimum task

#include <iostream>
#include <string>
using namespace std;

class Product {
  int id = 0;
  string name;
  double price = 0;
  int stock = 0;

public:
  bool init(int newId, const string& newName, double newPrice, int newStock) {
      // TODO: reject invalid data, accept only the first call
      id = newId;
      name = newName;
      price = newPrice;
      stock = newStock;
      return true;
  }
  bool changePrice(double newPrice) {
      // TODO
      return false;
  }
  bool sell(int qty) {
      // TODO
      return false;
  }
  bool restock(int qty) {
      // TODO
      return false;
  }
  double getValue() const {
      // TODO
      return 0;
  }
  int getStock() const { return stock; }
};

int main() {
  Product p;
  p.init(1, "Pen", 2.5, 10);
  cout << p.sell(3) << " " << p.getStock() << endl;    // expected: 1 7
  cout << p.sell(20) << " " << p.getStock() << endl;   // expected: 0 7
  cout << p.getValue() << endl;                        // expected: 17.5
}

bool init(int newId, const string& newName, double newPrice, int newStock) {
    if (id != 0) return false;                        // already initialised
    if (newId <= 0 || newName.empty() || newPrice <= 0 || newStock < 0) return false;
    id = newId;
    name = newName;
    price = newPrice;
    stock = newStock;
    return true;
}
bool changePrice(double newPrice) {
    if (newPrice <= 0) return false;
    price = newPrice;
    return true;
}
bool sell(int qty) {
    if (qty <= 0 || qty > stock) return false;
    stock -= qty;
    return true;
}
bool restock(int qty) {
    if (qty <= 0) return false;
    stock += qty;
    return true;
}
double getValue() const { return price * stock; }

Requirement change

New requirement: the warehouse holds at most 100 pieces of a product. A restock or init that would exceed 100 is refused.

• How many places in your code did you have to change? Why so few?
• What would you have had to search for in the starting program, with the public struct?
• Write void print(const Product& p) outside the class. Which methods can it call? What does the compiler say if getValue is not const?

Code in repository: L03

Romanian version