See if you can guess the output?

ABAP Insanity #1

FUNCTION ZTEST12345.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"  EXPORTING
*"     REFERENCE(E_TEST) TYPE  I
*"----------------------------------------------------------------------

  e_test = e_test + 1.

ENDFUNCTION.


Example A:

FUNCTION ZEXAMPLEA.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"  EXPORTING
*"     REFERENCE(E_TEST) TYPE  I
*"----------------------------------------------------------------------

  CALL FUNCTION 'ZTEST12345'
    IMPORTING
      E_TEST = e_test.

  CALL FUNCTION 'ZTEST12345'
    IMPORTING
      E_TEST = e_test.

ENDFUNCTION.


Example B:

FUNCTION ZEXAMPLEB.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"  EXPORTING
*"     REFERENCE(E_TEST) TYPE  I
*"----------------------------------------------------------------------

  CALL FUNCTION 'ZTEST12345'
*    IMPORTING
*      E_TEST = e_test
  .

  CALL FUNCTION 'ZTEST12345'
    IMPORTING
      E_TEST = e_test.

ENDFUNCTION.


Example C:

FUNCTION ZEXAMPLEC.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"  EXPORTING
*"     REFERENCE(E_TEST) TYPE  I
*"----------------------------------------------------------------------

  data e_test2 type i.

  CALL FUNCTION 'ZTEST12345'
    IMPORTING
      E_TEST = e_test2.

  CALL FUNCTION 'ZTEST12345'
    IMPORTING
      E_TEST = e_test.

ENDFUNCTION.


Example D:

FUNCTION ZEXAMPLED.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"  EXPORTING
*"     REFERENCE(E_TEST) TYPE  I
*"----------------------------------------------------------------------

  e_test = 1.

  CALL FUNCTION 'ZTEST12345'
    IMPORTING
      E_TEST = e_test.

ENDFUNCTION.



Answer key (value of e_test):

A: 2
B: 1
C: 1
D: 2

Moral of the story: The way to approximate functional programming in ABAP is using class 
methods and RETURNING parameters. But wait ... doesn't that just force pass-by-value, 
which the documentation tells us is incredibly inefficient for complex data structures? 
Yes, indeed it does.