Constructor and Class constructor

Question

Hi all
Can any one explain me the functionality about Constructor and class constructor??
As well as normal methods, which you call using CALL METHOD, there are two special methods
called CONSTRUCTOR and CLASS_CONSTRUCTOR, which are automatically called when you
create an object (CONSTRUCTOR) or when you first access the components of a class
(CLASS_CONSTRUCTOR)
I have read the above mentioned document from SAP Documents but i found it bit difficult to understand can any one please help me on this??
Thanks and Regards,
Arun Joseph

Answer 1

Hi,
Constructor
Implicitly, each class has an instance constructor method with the reserved name constructor and a static constructor method with the reserved name class_constructor.
The instance constructor is executed each time you create an object (instance) with the CREATE OBJECT statement, while the class constructor is executed exactly once before you first access a class.
The constructors are always present. However, to implement a constructor you must declare it explicitly with the METHODS or CLASS-METHODS statements. An instance constructor can have IMPORTING parameters and exceptions. You must pass all non-optional parameters when creating an object. Static constructors have no parameters.
Class Constructor
The static constructor is always called CLASS_CONSTRUCTER, and is called autmatically before the clas is first accessed, that is before any of the following actions are executed:
Creating an instance using CREATE_OBJECT
Adressing a static attribute using ->
Calling a ststic attribute using CALL METHOD
Registering a static event handler
Registering an evetm handler method for a static event
The static constructor cannot be called explicitly.
For better understanding check the following code:
REPORT  z_constructor.
      CLASS cl1 DEFINITION
CLASS cl1 DEFINITION.
  PUBLIC SECTION.
    METHODS:
      add,
      constructor IMPORTING v1 TYPE i
                            v2 TYPE i,
      display.
    CLASS-METHODS:
     class_constructor.
  PRIVATE SECTION.
    DATA:
    w_var1   TYPE i,
    w_var2   TYPE i,
    w_var3   TYPE i,
    w_result TYPE i.
ENDCLASS.                    "cl1 DEFINITION
      CLASS cl1 IMPLEMENTATION
CLASS cl1 IMPLEMENTATION.
  METHOD constructor.
    w_var1 = v1.
    w_var2 = v2.
  ENDMETHOD.                    "constructor
  METHOD class_constructor.
    WRITE:
    / 'Tihs is a class or static constructor.'.
  ENDMETHOD.                    "class_constructor
  METHOD add.
    w_result = w_var1 + w_var2.
  ENDMETHOD.                    "add
  METHOD display.
    WRITE:
    /'The result is =  ',w_result.
  ENDMETHOD.                    "display
endclass.
" Main program----
data:
  ref1 type ref to cl1.
parameters:
  w_var1 type i,
  w_var2 type i.
  start-of-selection.
  create object ref1 exporting v1 = w_var1
                              v2 = w_var2.
   ref1->add( ).
   ref1->d isplay( ).
Regards,
Anirban Bhattacharjee