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7.4 Procedures (7.4.1 Subroutine (Passing Parameters to Subroutines (three…

- - - - The parameter interface allows you to pass data to a subroutine and is defined by directly declaring variables on the FORM statement;
        these variables are called formal parameters.
        
        Formal parameters are filled by the PERFORM statement by maintaining the parameters in the PERFORM statement;
        such parameters are called actual parameters.
        
        The number of actual and formal parameters should always be same.
        
        While defining formal parameters, they can be either typed or untyped.
        
        untyped formal parameter
        
        can pass a variable of any data type or length
        
        typed parameter
        
        FORM abc using u1 type t. ENDFORM.
        
        more efficient than untyped parameters
  - - - PERFORM abc USING f1 f2 CHANGING f3.
        
        If the actual parameter is changed after the subroutine call, list it under CHANGING; otherwise, list it under USING.
    - - Parameters passed by reference (pass by reference parameters)
        
        When the parameters are passed by reference, the value of the actual parameter changed immediately the moment the contents of the formal parameter are changed
        
        If you don't want the value of an actual parameter to be changed automatically, you must pass it by value.
        
        use Using for input parameters (which are not changed in the subroutine)
        CHANGING for output parameters (which are changed in the subroutine)
      - Input parameters that pass values (pass by value parameters)
        
        FORM xyz USING VALUE (p1) CHANGING p2. ENDFORM.
        
        p1 passed by value and p2 passed by reference
        
        When you call the subroutine, the value of the actual parameter is passed to the formal parameter (it creates a copy of the original data). If the value of the formal parameter changes, this has no effect on the actual parameter.
      - Output parameters that pass values (pass by value and result parametes)
        
        FORM xyz USING VALUE (p1) CHANGING VALUE (p2). ENDFORM.
        
        contents of the actual parameter are copied to the formal parameter
        the actual parameter will be changed after the subroutine call
        the contents of the formal parameter are copied to the actual parameter either once ENDFORM is executed or when the subroutine is terminated with the CHECK or EXIT statements
  - - - when referring to a structure, use the syntax TYPE TABLE OF
        
        FORM abc USING pt_makt TYPE STANDARD TABLE OF ty_markt. ENDFORM.
      - when a data object is referred to a table type using the TYPE addition, it creates an internal table
- - - - used for local modularization
    - - are created globally and are used for global modularization
    - - support object-oriented programming concepts and contain the functions of classes and their instances in ABAP objects
      - defined in classes and can be maintained locally in the program or globally in the system