C++ Objects

Creating Objects

An object is an instance of a class. You can create many objects from the same class.

#include <iostream>
using namespace std;

class Rectangle {
  public:
    double width;
    double height;

    double area()      { return width * height; }
    double perimeter() { return 2 * (width + height); }
};

int main() {
    Rectangle r1, r2;

    r1.width = 5.0; r1.height = 3.0;
    r2.width = 8.5; r2.height = 4.2;

    cout << "R1 Area: "      << r1.area()      << endl;
    cout << "R1 Perimeter: " << r1.perimeter() << endl;
    cout << "R2 Area: "      << r2.area()      << endl;
    return 0;
}

Output:

R1 Area: 15
R1 Perimeter: 16
R2 Area: 35.7

The this Pointer

Inside a method, this is a pointer to the current object:

class Box {
  public:
    int size;
    void setSize(int size) {
        this->size = size;   // 'this->size' is the member; 'size' is the parameter
    }
};

#include <iostream>
using namespace std;

class BankAccount {
  public:
    string owner;
    double balance;
    void deposit(double amount)  { balance += amount; }
    void withdraw(double amount) { if (amount <= balance) balance -= amount; }
    void printBalance()          { cout << owner << ": $" << balance << endl; }
};

int main() {
    BankAccount acc;
    acc.owner = "Alice"; acc.balance = 1000.0;
    acc.deposit(500);
    acc.withdraw(200);
    acc.printBalance();   // Alice: $1300
    return 0;
}

Summary

• Objects are instances of classes created using the class name as a type.
• Each object has its own copy of member variables.
• this is a pointer to the current object inside a method.