Working with Field Symbols

An important, but frequently misunderstood aspect of ABAP, is 
the "Field Symbol". But you'll find they aren't mysterious. 
In fact, they may remind you of some features in popular 
general-purpose programming languages.

Field symbols allow you to:
**	Assign an alias to a data object(for example, a shortened 
        name for data objects structured through several hierarchies
        - <fs>-f instead of rec1-rec2-rec3-f)
**	Set the offset and length for a string variably at runtime
**	Set a pointer to a data object that you determine at runtime (dynamic ASSIGN)
**	Adopt or change the type of a field dynamically at runtime 
**	Access components of a structure
**	(from Release 4.5A) Point to lines of an internal table 
        (process internal tables without a separate work area)

Field symbols in ABAP are similar to pointers in other programming 
languages. However, pointers (as used in PASCAL or C) differ from ABAP
field symbols in their reference syntax.

The statement ASSIGN f to <fs> assigns the field f to field
symbol <fs>. The field symbol <fs> then "points" to the
contents of field f at runtime. This means that all changes to the 
contents of f are visible in <fs> and vice versa. You declare
the field symbol <fs> using the statement FIELD-SYMBOLS: <fs>.


Reference syntax
Programming languages such as PASCAL and C use a dereferencing symbol
to indicate the difference between a reference and the object to which
it refers; so PASCAL would use p^ for a pointer instead of p, C would 
use *p instead of p. ABAP does not have any such dereferencing symbol.

**	In PASCAL or C, if you assign a pointer p1 to a pointer p2, 
you force p1 to point to the object to which p2 refers (reference semantics).

**	In ABAP, if you assign a field symbol <fs1> to a field 
symbol <fs2>, <fs1> takes the value of the data object to
which <fs2> refers (value semantics).

**	Field symbols in ABAP are always dereferenced, that is, 
they always access the referenced data object. If you want to 
change the reference yourself in ABAP, you can use the ASSIGN statement
to assign field symbol <fs1> to field symbol <fs2>.


Using field symbols
You declare field symbols using the FIELD-SYMBOLS statement. 
They may be declared either with or without a specific type.
At runtime you assign a field to the field symbol using the ASSIGN 
statement. All of the operations on the field symbol act on the field
assigned to it.
When you assign a field to an untyped field symbol, the field symbol 
adopts the type of the field. If, on the other hand, you want to assign 
a field to a typed field symbol, the type of the field and that of the 
field symbol must be compatible.
A field symbol can point to any data object and from Release 4.5A, 
they can also point to lines of internal tables.

The brackets (<>) are part of the syntax.

Use the expression <fs> IS ASSIGNED to find out whether the field
symbol <fs> is assigned to a field.
The statement UNASSIGN <fs> sets the field symbol <fs> so
that it points to nothing. The logical expression <fs> 
IS ASSIGNED is then false. The corresponding negative expression 
is IF NOT <fs> IS ASSIGNED.

An unassigned field symbol <fs> behaves as a constant with 
type C(1) and initial value SPACE.
MOVE <fs> 
TO dest	Transfers the initial value SPACE to the variable dest
MOVE 'A' to <fs>	
Not possible, since <fs> is a constant 
(runtime error).

To lift a type restriction, use the CASTING addition in the
ASSIGN statement. The data object is then interpreted as though
it had the data type of the field symbol. You can also do this
with untyped field symbols using the CASTING TYPE <type> addition.

The danger with pointers is that they may point to invalid areas. 
This danger is not so acute in ABAP, because the language does not 
use address arithmetic (for example, in other languages, pointer p 
might point to address 1024. After the statement p = p + 10, it would
point to the address 1034). However, the danger does still exist, and
memory protection violations lead to runtime errors.

A pointer in ABAP may not point beyond a segment boundary. ABAP does
not have one large address space, but rather a set of segments. 
Each of the following has its own segment:
*	All global data
*	All local data
*	Each table work area (TABLES)
*	Each COMMON PART

You should only let field symbols move within an elementary field or
structure where ABAP allows you to assign both within the global data
and beyond a field boundary.