Data types and data objects are the essential components of the ABAP type concept. Both can be declared and maintained by the user. This contrasts with other programming languages like C, FORTRAN, or PASCAL where the user can declare data objects but is restricted to the predefined data types. In ABAP, you can process data types independently of data objects, using a syntax which is parallel to normal data declaration. It is also possible to store data types centrally outside a specific program.

The following list is an overview of the main features of data types and objects:

• Data types (elementary or structured)

• Data types are pure descriptions.
• They do not occupy memory.
• They describe the technical attributes of data objects.
• The data type is an attribute of a data object and is closely connected with it.

• Data objects (literals, variables, constants, and so on):

• Data objects are the physical units a program uses at runtime.
• Each data object has a specific data type assigned to it.
• Each data object occupies some space in memory.
• ABAP processes a data object according to its data type.

In your programs, you must declare all the data objects you want to work with. In the declaration, you must assign certain attributes to a data object. The most important of these attributes is the data type. In ABAP, you can either use predefined data types similar to other program languages, or user-defined data types.

User-defined data types are a powerful programming tool, allowing you a high degree of flexibility. They range from elementary types (such as character strings of a given length) to very complex structures (like nested tables).

Assigning user-defined data types to data objects allow you to work with precisely the data objects you require. User-defined data types can be used in the same way as predefined data types. You can declare them locally within a program or store them globally in the ABAP Dictionary.

There are three hierarchical levels of data types and objects:

• program-independent data, defined in the ABAP Dictionary

• internal data used globally in one program

• data used locally in a procedure (subroutine, function module)

You will learn more about these levels in the corresponding sections of this guide.