Field symbols are placeholders or symbolic names for other fields. They do not physically reserve space for a field, but point to its contents. A field symbol cam point to any data object. The data object to which a field symbol points is assigned to it after it has been declared in the program.
Whenever you address a field symbol in a program, you are addressing the field that is assigned to the field symbol. After successful assignment, there is no difference in ABAP whether you reference the field symbol or the field itself. You must assign a field to a field symbol before you can address it in a program.
Field symbols are similar to de-referenced pointers in the C programming language (that is, pointers to which the content operator * is applied). In ABAP, data references represent a real equivalent to pointers in the sense of variables that contain a memory address and can be used without the contents operator.
All operations programmed with field symbols are applied to the field assigned to it. A MOVE statement between two field symbols, for example, assigns the contents of the field assigned to another source field symbol to the field assigned to the target field symbol. The field symbols themselves point to the same fields after the MOVE statement as they did before the MOVEstatement.
You can create field symbols either without or with type specifications. If you do not specify a type, the field symbol inherits all of the technical attributes of the field assigned to it. If you do specify a type, the system checks during the field assignment whether the assigned field matches the type of field symbol.
Field symbols provide greater flexibility when you address data objects:
· You can assign one field symbol to another, which allows you to address subfields.
· Assignments to field symbols may extend beyond field boundaries. This allows you to address regular sequences of fields in memory efficiently.
· You can also force a field symbol to take different technical properties than those of the field assigned to it (casting).
The flexibility of field symbols provides elegant solutions to certain problems. On the other hand, it does mean that errors can easily occur. Since fields are not assigned to field symbols until runtime, the effectiveness of syntax and security checks is very limited for operations involving field symbols. This can lead to runtime errors or incorrect data assignments.
While runtime errors indicate an obvious problem, incorrect data assignments are dangerous because they can be very difficult to detect. For this reason, you should only use field symbols if you cannot achieve the same result using other ABAP statements.
For example, you may want to process part of a string where the offset and length depend on the contents of the field. You could use field symbols in this case. The MOVE statement (with your own auxiliary variables, if required) is much safer than using field symbols, since it cannot address memory beyond the boundary of a field. However, field symbols may improve performance in some cases.