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Field Guide

Objects

Once we start working with member functions, we are moving into object-oriented programming. So…

What are objects?

The values we create from a struct with member functions are known as objects.

Illustration of objects

Example

The following code creates seven text_message objects. The variables m1 to m5 are each an object on the stack, which in C++ you interact with using the dot operator (.). The m6 variable is a pointer that can point to a text_message object. We initially create one message object, and interact with it, before deleting it. We can then reuse that pointer and create a new text_message object for it to refer to.

#include "splashkit.h"


struct text_message
{
  string text;   ///< The content of the message.
  string sender; ///< The sender of the message.


  text_message(string sender, string text)
  {
    this->sender = sender;
    this->text = text;
  }


  text_message()
  {
    sender = "";
    text = "";
  }


  void print()
  {
    write_line("Message from " + sender + ": " + text);
  }
};


/**
 * @brief Entry point for the message program. Demonstrates usage of the
 * text_message struct.
 */
int main()
{
  // Create a text_message object - m1 - on the stack. It has name "John" and text "Hello"
  text_message m1("John", "Hello");


  // Create a text_message object - m2 - on the stack. It has name "Mary" and text "Hi"
  text_message m2("Mary", "Hi");


  // m3, m4, and m5 are also text_message objects
  text_message m3("John", "How are you?");
  text_message m4("Mary", "I'm good thanks");
  text_message m5;


  // You can interact with the objects...
  m1.print(); // ask m1 to print itself
  m2.print(); // ask m2 to print itself
  m3.print();
  m4.print();
  m5.print();


  // Objects can also be on the heap... in this case m6 is a pointer to the object.
  // the object itself is on the heap
  text_message *m6 = new text_message("John", "I'm glad to hear it");


  // With pointer, use the -> operator to interact with the object
  m6->print();  // ask the object that m6 points to, to print itself
  delete m6; // now destroy the object that m6 refers to...


  // The object referred to by m6 no longer exists... but
  m6 = NULL; // m6 still exists - it is a pointer. We can change it to point to null now.


  // We can even create another object for m6 to refer to
  m6 = new text_message("Ye", "Howdy");
  delete m6;


  return 0;
}