Objects

Chapter 3: Objects

Lecture Goals

To become familiar with objects
To learn about the properties of several sample classes that were designed for this book
To be able to construct objects and supply initial values
To understand member functions and the dot notation
To be able to modify and query the state of an object through member functions

Constructing Objects

An object is a value that can be created, stored and manipulated in a programming language.

	string greeting = "Hello";
	cout << greeting.substr(0,4);

Every object in C++ must belong to a class.
A class is a data type (like int or double) that is programmer defined, whereas int and double are defined by the designers of the C++ language.
We will learn to work with the class Time and the class Employee.
- Remember, these are programmer defined and not part of standard C++.

Suppose you want to know how many seconds will elapse between now and midnight.
To use the Time class we must include the file that contains its definition:
```
#include "ccc_time.h"
```

We use " " instead of < > because the file is not a standard (system) header.
The name stands for Computing Concepts with C++ Essentials.

Creating any object is called construction.
We can construct an object just like we create any variable.
```
Time sometime;
double interest_rate;
string greeting;
```

We initialize an object by passing construction parameters when the object is created.

Time day_end = Time(23, 59, 59); /* the last second of the day */
double interest_rate = 3.5;
string greeting = "Hello";

The shorthand in this very common situation is:

Time day_end(23, 59, 59); /* the last second of the day */

Creating objects without parameters is called default construction. No parameters means not parenthesis! The Time
class creates an object with the current time.
```
Time now = Time(); /* the time this object is created */
Time now;          /* the time this object is created */
Time later();      /* NO! */
```
An object can be created anytime by supplying the class name with (or without) parameters. The assignment operator changes the value of the object variable
.
```
sometime = Time(12, 5, 18); /* we already declared sometime */
```

Syntax 3.1: Constant Object Construction

Class_name(construction parameters);

Example:	Time(19, 0, 0);
Purpose:	Construct a new object for use in an expression.

Syntax 3.2: Object Variable Definition

Class_name variable_name(construction parameters);

Example:	Time homework_due(19, 0, 0);
Purpose:	Define a new object variable and supply parameter values for initialization.

Using Objects

A function applied to an object with the dot notation is called a member function.

greeting.length();    /* returns the length of greetings */
greeting.substr(0,1); /* returns a substring */

The Time class has several member functions used to find the state of the object:

now.get_seconds(); /* returns the seconds value of now */
now.get_minutes(); /* returns the minutes value of now */
now.get_hours();   /* returns the hours value of now */

You can add a certain number of seconds to the time:

now.add_seconds(1000); /* add 1000 seconds to the time stored in now */

Using Objects (`time1.cpp`)

The Time class does not have any member functions used to set the state of the object to avoid abuse:

now.set_hours(2); /* NO! Not a supported member function */
now.set_hours(9999); /* NO! Doesn't make sense */

The Time class does provide other member functions to facilitate other actions:

now.add_seconds(1000); /* Changes now to move by 1000 seconds */
now.seconds_from(day_end); /* Computes number of seconds between now and day_end */

Using Objects (`time2.cpp`)

Real-Life Objects

Real-Life Objects (`employee.cpp`)

Object-oriented programming allows us to easily model entities from real life.
As an example, the text provides a simple Employee class:
- All employees have names and salaries which are set at construction.
```
Employee harry("Hacker, Harry", 45000.00);
```
- Both attributes can be queried.
```
cout << "Name: " << harry.get_name() << "\n";
cout << "Salary: " << harry.get_salary() << "\n";
```
- Only salary can be set.
```
harry.set_salary(new_salary);
```