Subroutine parameters ..by default pass by value or pass by ref

Question

how parameters pass ?  value or ref
if we use changing in both perform and form...will it become CALL BY REF?
Message was edited by:
        balaji velpuri

Answer 1

Hi,
<u><b>The Parameter Interface</b></u>
The USING and CHANGING additions in the FORM statement define the formal parameters of a subroutine. The sequence of the additions is fixed. Each addition can be followed by a list of any number of formal parameters. When you call a subroutine, you must fill all formal parameters with the values from the actual parameters. At the end of the subroutine, the formal parameters are passed back to the corresponding actual parameters.
Within a subroutine, formal parameters behave like dynamic local data. You can use them in the same way as normal local data objects that you would declare with the DATA statement. They mask global data objects with the same name. The value of the parameters at the start of the subroutine is the value passed from the corresponding actual parameter.
Subroutines can have the following formal parameters:
<u><b>Parameters Passed by Reference</b></u>
You list these parameters after USING or CHANGING without the VALUE addition:
FORM <subr> USING ... <pi> [TYPE <t>|LIKE <f>] ...
CHANGING ... <pi> [TYPE <t>|LIKE <f>] ...
The formal parameter occupies no memory of its own. During a subroutine call, only the address of the actual parameter is transferred to the formal parameter. The subroutine works with the field from the calling program. If the value of the formal parameter changes, the contents of the actual parameter in the calling program also change.
For calling by reference, USING and CHANGING are equivalent. For documentation purposes, you should use USING for input parameters which are not changed in the subroutine, and CHANGING for output parameters which are changed in the subroutine.
To avoid the value of an actual parameter being changed automatically, you must pass it by value.
<u><b>Input Parameters That Pass Values</b></u>
You list these parameters after USING with the VALUE addition:
FORM <subr> USING ... VALUE(<pi>) [TYPE <t>|LIKE <f>] ...
The formal parameter occupies its own memory space. When you call the subroutine, the value of the actual parameter is passed to the formal parameter. If the value of the formal parameter changes, this has no effect on the actual parameter.
<u><b>Output Parameters That Pass Values</b></u>
You list these parameters after CHANGING with the VALUE addition:
FORM <subr> CHANGING ... VALUE(<pi>) [TYPE <t>|LIKE <f>] ...
The formal parameter occupies its own memory space. When you call the subroutine, the value of the actual parameter is passed to the formal parameter. If the subroutine concludes successfully, that is, when the ENDFORM statement occurs, or when the subroutine is terminated through a CHECK or EXIT statement, the current value of the formal parameter is copied into the actual parameter.
If the subroutine terminates prematurely due to an error message, no value is passed. It only makes sense to terminate a subroutine through an error message in the PAI processing of a screen, that is, in a PAI module, in the AT SELECTION-SCREEN event, or after an interactive list event.
<u><b>Specifying the Type of Formal Parameters</b></u>
Formal parameters can have any valid ABAP data type. You can specify the type of a formal parameter, either generically or fully, using the TYPE or LIKE addition. If you specify a generic type, the type of the formal parameter is either partially specified or not specified at all. Any attributes that are not specified are inherited from the corresponding actual parameter when the subroutine is called. If you specify the type fully, all of the technical attributes of the formal parameter are defined with the subroutine definition.
The following remarks about specifying the types of parameters also apply to the parameters of other procedures (function modules and methods). If you have specified the type of the formal parameters, the system checks that the
corresponding actual parameters are compatible when the subroutine is called. For internal subroutines, the system checks this in the syntax check. For external subroutines, the check cannot occur until runtime. By specifying the type, you ensure that a subroutine always works with the correct data type. Generic formal parameters allow a large degree of freedom when you call subroutines, since you can pass data of any type. This restricts accordingly the options for processing data in the subroutine, since the operations must be valid for all data types. For example, assigning one data object to another may not even be possible for all data types. If you specify the types of subroutine parameters, you can perform a much wider range of operations, since only the data appropriate to those operations can be passed in the call. If you want to process structured data objects component by component in a subroutine, you must specify the type of the parameter.
Regards,
Bhaskar