Parameter passing by value or by reference in function module

Question

hi everybody:
Im a beginner for abap.
Below description is described in online help.
In function module, the CALL FUNCTION statement can pass import, export, and changing parameters either by value or by reference. Table parameters are always transferred by reference.
I understand parameters passing by value means values carried by parameters are transferred, but I do not understand what is "by reference".
Please kindly give me a explanation.
Regards.
Andy

Answer 1

hi,
Function modules are modular units with interfaces. The interface can contain the following elements:
Import parameters are parameters passed to the function module. In general, these are assigned
standard ABAP Dictionary types. Import parameters can also be characterized as optional.
Export parameters are passed from the function module to the calling program. Export parameters
are always optional and for that reason do not need to be accepted by the calling program.
Changing parameters are passed to the function module and can be changed by it. The result is
returned to the calling program after the function module has executed.
Exceptions are used to intercept errors. If an error triggers an exception in a function module, the
function module stops. You can assign exceptions to numbers in the calling program, which sets the
system field SY-SUBRC to that value. This return code can then be handled by the program.
By reference Passes a pointer to the original memory location.  Very efficient
By value Allocates a new memory location for use within the subroutine. The memory is freed when the subroutine ends.  Prevents changes to passed variable
By value and result Similar to pass by value, but the contents of the new memory is copied back into the original memory before returning.  Allows changes and allows a rollback
When you pass a parameter by reference, new memory is not allocated for the value. Instead, a pointer to the original memory location is passed. All references to the parameter are references to the original memory location. Changes to the variable within the subroutine update the original memory location immediately.
1  report ztx1804.
2  data f1 value 'A'.
3
4  perform s1 using f1.
5  write / f1.
6
7  form s1 using p1.
8      p1 = 'X'.
9      endform.
The code in Listing produces the following output:
X
Hope this helps, Do reward.