Skip to main content

Constructors and Destructors of Derived Classes

Classes can have Constructors and/or Destructors and Derived Classes are no different.

Situation remains understandable until both the base and its derived class have Constructors and/or Destructors. Since the derived class contains more than one Constructors and/or Destructors, it becomes confusing which one will be called when.

This is because when an object the inherited class is constructed both the constructors (base’s and its own) should be invoked and same applies when it gets destructed.

This article will clear all this!

Consider the following example program:


  // -- INHERITANCE --
  // Constructors, Destructors
  // and Inheritance
  #include<iostream.h>

  // base class
  class base
  {
  public:
    base(){cout<<"Constructing Base\n";}
    ~base(){cout<<"Destructing Base\n";}
  };

  // derived class
  class derived:public base
  {
  public:
    derived(){cout<<"Constructing Derived\n";}
    ~derived(){cout<<"Destructing Derived\n";}
  };

  void main(void)
  {
    derived obj;

    // do nothing else, only
    // construct and destruct
    // the inherited class object
  }

OUTPUT:

  Constructing Base
  Constructing Derived
  Destructing Derived
  Destructing Base
  Press any key to continue

So here is the general rule:

Constructors are called in the order of derivation and Destructors in the reverse order.

One more example will clear the confusions, if any.:


  // -- INHERITANCE --
  // Constructors, Destructors
  // and Inheritance
  #include<iostream.h>

  // base class (1)
  class base
  {
  public:
    base(){cout<<"Constructing Base\n";}
    ~base(){cout<<"Destructing Base\n";}
  };

  // derived class
  // derived form 'base'
  class derived1:public base
  {
  public:
    derived1(){cout<<"Constructing Derived1\n";}
    ~derived1(){cout<<"Destructing Derived1\n";}
  };

  // derived from a derived class
  // 'derived1'
  class derived2:public derived1
  {
  public:
    derived2(){cout<<"Constructing Derived2\n";}
    ~derived2(){cout<<"Destructing Derived2\n";}
  };

  void main(void)
  {
    derived2 obj;

    // do nothing else, only
    // construct and destruct
    // the inherited class object
  }

OUTPUT:

  Constructing Base
  Constructing Derived1
  Constructing Derived2
  Destructing Derived2
  Destructing Derived1
  Destructing Base
  Press any key to continue 

Related Articles:

Popular posts from this blog

Fix For Toshiba Satellite "RTC Battery is Low" Error (with Pictures)

RTC Battery is Low Error on a Toshiba Satellite laptop "RTC Battery is Low..." An error message flashing while you try to boot your laptop is enough to panic many people. But worry not! "RTC Battery" stands for Real-Time Clock battery which almost all laptops and PCs have on their motherboard to power the clock and sometimes to also keep the CMOS settings from getting erased while the system is switched off.  It is not uncommon for these batteries to last for years before requiring a replacement as the clock consumes very less power. And contrary to what some people tell you - they are not rechargeable or getting charged while your computer or laptop is running. In this article, we'll learn everything about RTC batteries and how to fix the error on your Toshiba Satellite laptop. What is an RTC Battery? RTC or CMOS batteries are small coin-shaped lithium batteries with a 3-volts output. Most laptops use

The Best Way(s) to Comment out PHP/HTML Code

PHP supports various styles of comments. Please check the following example: <?php // Single line comment code (); # Single line Comment code2 (); /* Multi Line comment code(); The code inside doesn't run */ // /* This doesn NOT start a multi-line comment block /* Multi line comment block The following line still ends the multi-line comment block //*/ The " # " comment style, though, is rarely used. Do note, in the example, that anything (even a multi-block comment /* ) after a " // " or " # " is a comment, and /* */ around any single-line comment overrides it. This information will come in handy when we learn about some neat tricks next. Comment out PHP Code Blocks Check the following code <?php //* Toggle line if ( 1 ) {      // } else {      // } //*/ //* Toggle line if ( 2 ) {      // } else {      // } //*/ Now see how easy it is to toggle a part of PHP code by just removing or adding a single " / " from th

Designing a Simple Order Form Application in PHP

Ok guys, for this post we’re going to create a simple yet complete order form page. Order forms are used on many sites to take customers order online. Order forms should have the capability to take orders from visitors regarding what items they want to purchase and store the information for further processing. For this post’s example, we are going to create an order form for a Book Seller. The form will be designed to take order of five different items (books). Our order form application should be able to take order of five different items in any separate quantities tht user wants, it should also ask for shipping address and name of the customer. It should then store the information provided in a file along with the date and time order was placed. The application should also be able to take any number of orders and store them all linearly for further human processing. For this, we need a front end of a HTML form to which the user would interact a