ARTICLE
Internal Tables with Header Line
Outside classes - if an internal table is not a component of a structure
or a row in another internal table - it is still possible to create an
internal table with a header line.
Declaration of Header Lines
Header lines of internal tables are created
by using the addition WITH HEADER LINE
of the statement DATA to declare internal tables,
when using the obsolete statement string
DATA - BEGIN OF OCCURS to declare structured standard
tables,
when using the obsolete statement RANGES to
declare ranges tables ,
when declaring selection tables with
the statement SELECT-OPTIONS ,
when using table parameters for
function modules and
subroutines .
Properties of Header Lines
A header line is a work area whose
data type is the same as the row type of the internal table
whose name is the same as the name of the internal table.
If a header line exists, therefore, an ABAP program includes two data
objects with the same name (the actual internal table and the header
line). The internal table and header line are accessed as follows:
Many processing statements for internal tables have
obsolete short forms in which the header
lines is used as an implicit work area if no explicit work area is
specified.
In all other cases, the statement and
operand position decide whether the table
body or the header line is used when the table name is specified. The
header line is the usual choice. The name (and just the name) of an
internal table with header line is interpreted as the table body only in
the following cases:
Operand positions in the processing
statements for internal tables in which the internal table to be
processed is specified.
Internal table specified in a table
expression
When saving and reading data clusters
using EXPORT and
IMPORT .
In the statement FREE .
In the obsolete statement SEARCH .
When a token is specified dynamically in
Open SQL (except when database tables are specified).
To force access to the table body in any operand position when a header
line exists, square brackets can be specified directly after the name of
an internal table in all operand positions (for example, itab[] ).
This does not apply, however, when specifying the internal table in a
table expression .
Notes
A field symbol or a data reference can only address either the table
body or the header line. This means that a field symbol or a data
reference is never ambiguous.
Both the table body and the header line are passed when a table with
header line is passed to table
parameters .
[] can be specified for internal tables without header line, but
does not need to be. This is because the name (without [] ) of an
internal table without header line is interpreted as the table body in
all operand positions regardless.
In many operand positions that expect internal tables, the syntax check
forces [] to be specified after the name of an internal table
with header line.
Use
The use of header lines is highly error-prone, due to the repeated use
of one name for two data objects. If at all possible, avoid creating and
using header lines (even outside of classes).
Do not use the addition WITH HEADER LINE
and the statement string DATA
- BEGIN OF OCCURS any more and do not use
RANGES to declare the ranges tables.
Avoid using table parameters if at all possible.
In cases where a header line absolutely has to be created (such as in
selection tables or in procedures that still require table parameters
(generally only remote-enabled
function modules )), never work with the header line and use the
additional explicitly work areas declared explicitly instead.
Notes
A work area for replacing a header line can be declared very simply by
using the addition LINE OF of the statements TYPES ,
DATA , and so on.
Do not mistake the use of an additional work area with the explicit
completion of the implicit short forms ,
such as LOOP AT itab INTO itab . The latter case is one of the
undesired ways to use a header line.
Tables with header lines do not offer any performance advantages.
Example
The following example shows a typical instance of handling internal
tables with header lines: An internal table with header line (here the
table parameter of a function module) is initialized using
CLEAR , but the [] is not appended to the
name. In this case, only the header line is deleted, which is not
usually noticed until runtime.
FUNCTION work_with_tables.
*"---------------------------------
*"*"Local Interface
*" TABLES
*" table STRUCTURE structure
*"----------------------------------
CLEAR table.
...
ENDFUNCTION.
Documentation extract taken from SAP system, � Copyright SAP AG. All rights reserved
