ABAP Keyword Documentation → ABAP Programming Guidelines → Structure and Style → Comments
Arrangement in the Source Code
Other versions: 7.31 | 7.40 | 7.54
Background
The arrangement of comments plays an important role (in addition to comment language and comment content) in making programs easy to read.
Rule
Arrange comments correctly
Place comments in front of the statements that they describe. The horizontal arrangement of comments should follow the indentations of the source code. End of line comments should only be placed after declarative or concluding statements.
Details
Vertical positioning
For control structures, this means that comment lines directly before a control statement (such as
IF or WHILE) refer to the associated condition and comment lines after
the control statement refer to the associated statement block. Comment lines directly before an
ELSE or WHEN OTHERS statement have obviously been put in the wrong place.
End of line comments
Therefore, you should make comments for entire statement blocks. Use self-contained comment lines to do this. This is because it is difficult to describe a reference to more than one statement line if you use end of line comments.
End of line comments are suitable for the following situations:
- To comment on declarative statements
- To indicate block ends (separate from indentations) in larger control structures
- To justify pseudo comments (at the relevant places) for hiding messages from the extended program check or Code Inspector.
The pseudo comments for hiding warnings of the extended program check and the Code Inspector play a special role. They are not comments in the usual sense of the word. They are program directives that must be appear in the same lines as the commented statements to take full effect. These pseudo comments were replaced by pragmas for the extended program check.
Indentations
These indentations can only be achieved using comments that start with a quotation mark ("), because this character can be in any position. A comment line that starts with an asterisk (*) must always be in the first position. It is therefore strongly recommended that you start all comments used in procedures (methods) with a quotation mark and the correct indentation. Comment lines that start with a quotation mark must not be confused with end of line comments, which are appear after different code.
Comment lines that start with an asterisk should only be used for head comments of classes and procedures. Here they help to subdivide a source code into logical sections. In addition, they are useful for temporarily disabling statements by commenting them out. The commented-out code can be clearly distinguished from actually indented comments.
Bad example
The following source code shows the implementation part of a class. The positioning of the comments does not follow the above rule.
CLASS application IMPLEMENTATION. "Application class
METHOD main. "Main Method
DATA items TYPE STANDARD TABLE
OF REF TO item.
DATA item_ref LIKE LINE OF items.
* Amount data
DATA amount TYPE i.
DATA total_amount TYPE i.
...
LOOP AT items INTO item_ref. "Loop over all items
IF item_ref IS BOUND AND
item_ref->is_valid( ) = abap_true. "Check validity
amount = item_ref->get_amount( ). "Get amount
ADD amount TO total_amount. "Add amount to totals
... "...
ELSE.
...
ENDIF.
ENDLOOP.
...
ENDMETHOD.
ENDCLASS.
Good example
The following source code shows the same implementation part as above. However, the comments are positioned as recommended. Comment lines that start with an asterisk (*) are used as header comments in the program structure. End of line comments only appear after declarations and block ends. All other comments appear in comment lines before the described statements and are indented accordingly.
*----------------------------------------------------------*
* Class implementations
*
*----------------------------------------------------------*
CLASS application IMPLEMENTATION.
METHOD main.
DATA: items TYPE STANDARD TABLE
OF REF TO item, "Item table
item_ref LIKE LINE OF items. "Item reference
DATA: amount TYPE i, "Amount per item
total_amount TYPE i. "Total amount of items
...
LOOP AT items INTO item_ref.
IF item_ref IS BOUND AND
item_ref->is_valid( ) = abap_true.
"Compute total amount for valid items
amount = item_ref->get_amount( ).
ADD amount TO total_amount.
...
ELSE.
ENDIF. "item_ref IS BOUND AND...
ENDLOOP.
...
ENDMETHOD. "main
*----------------------------------------------------------*
ENDCLASS. "application