Defining classes and structs in our programs are a way that we can create our own data types for our variables. When we define our own structures, we can create them with internal variables and functions available to them. The variables we create, whose data types come from a defined class or struct, is known as an object.

Design-wise, the idea is to take real-world objects and find a way to represent them in a computer program as an object that has attributes (variables) and functionality (functions).

The concept of Object Oriented Programming is creating a design that is easy to maintain over time. In particular, there are some core design goals behind OOP, including:

• Encapsulation: Giving the user / other programmers an “interface” \ to interact with the objects, hiding the inner-workings within the class.
  • Certain functions are made public, which other programmers can use to interface with the object.
  • The other programmers don't need to worry about the inner-workings of the object in order to use it.
  • The developer of the class can modify how the internals work without breaking the public interface.
  • Helps protect the data within the class from being accessed or modified by external things that shouldn't have access to it.
• Loose coupling: Ideally, different objects in a program shouldn't have their functionality tied to other objects too closely; we want to reduce inter-dependence between objects. When objects are more independent from each other, we say they are loosely coupled.
• High cohesion: When we design our objects, we shouldn't just throw everything and the kitchen sink into one object. To design an object with high cohesion means that everything inside the object belongs to that object - reduce the clutter.

A basic struct declaration is of the form…

#ifndef _MYFILE_H
#define _MYFILE_H

struct STRUCTNAME
{
  // member variables
  int MEMBER1;
  string MEMBER2;
  float MEMBER3;
};

#endif

Note that the closing curly-brace } must end with a ; otherwise the compiler will give you errors.

You can name your struct anything you'd please but it has to abide by the C++ naming rules (no spaces, no keywords, can contain letters, numbers, and underscores).

Any variables we declare within the struct are known as member variables - they are members of that struct.

Any new variables you declare whose data type is this struct will have its own copy of each of these variables.

A object is a type of variable whose data type is a struct (or class). To declare a variable whose data type is a struct, it looks like any normal variable declaration:

DATATYPE VARIABLE;

Our struct object variables each have their own member variables, which we access via the dot operator .. We can assign values to it, our read its value, similar to any normal variable.

VARIABLE.MEMBER1 = 10;
VARIABLE.MEMBER2 = "ASDF";
VARIABLE.MEMBER3 = 2.99;

cout << VARIABLE.MEMBER1 << endl;

When we have a struct declared and an object-variable created…

Book myBook;

… We won't be able to use cout on the object…

cout << myBook << endl;

… (Unless we overload operators, but that's for a future class!) Instead, remember that you have to output each member variable in the way you wish, because C++ won't know how you want to display the contents of a struct!…

cout << "Title:  " << myBook.title << endl
     << "Author: " << myBook.author << endl;