Validity and Visibility
The context of a declarative statement in the source code of a program determines the validity and visibility of the declared component. In ABAP programs, there are three contexts in which data types and data objects can be declared:
- Locally in procedures
Local data in procedures is valid while a procedure is being executed. It is created when the procedure is called, and deleted when the procedure is exited. In addition to their local data and formal parameters, procedures also 'see' the global data of the master program. Methods also 'see' the components of their class.
- As components of classes
Static attributes of classes exist for as long as the internal session of the ABAP program exists. Instance attributes of classes are related to the lifetime of objects. They are created when an object is instantiated and deleted along with the object by the garbage collector. Classes also see the global data of the master program.
- Globally in the master program
Global program data exists as long as the internal session of the ABAP program exists. The data is created when the program is loaded in the internal session, and deleted from the internal session when the program is removed. An ABAP program normally only sees its own global data (with the exception of interface work areas).
All contexts see the cross-program type definitions in ABAP Dictionary and the type definitions and data declarations in the public visibility sections of global classes. Local declarations, however, obscure global declarations with the same name. In statements in which it is possible to specify a data type, a class, or an interface dynamically, the obscuring of a global type by a local type can be canceled by specifying an absolute type name.
In addition to context-dependent visibility, the position of the declaration in the source code also plays a role. In a statement of an ABAP program, only previous declarations are visible, regardless of the context. To enable the order in the program to match the context-dependent visibility, all global declarations of a program should be specified at the start of the source code, before the first processing block, and all local declarations should be specified directly after the introduction of a procedure. Viewed like this, the definition of an interface and the declaration part of a class are both global declarations. It must be ensured that reference variables that reference a class are not declared until after the class has been declared. The implementation part, on the other hand, is one of the processing blocks.
- All data declared in the declaration part of an ABAP program is global to the program. With the
exception of the event blocks
AT SELECTION-SCREEN, all data declared within event blocks and dialog modules is also global to the program. The data declared in the above event blocks is local to these event blocks. Data that is declared between discrete processing blocks is also global to the program.
Declares a program-global field
g_field, an instance attribute
of a class, and a local field
field of a method. Reference is made to the outer declarations in the inner declarations.
DATA g_field TYPE string VALUE 'global field'. CLASS cls DEFINITION. PUBLIC SECTION. DATA attr LIKE g_field VALUE 'instance attribute'. METHODS meth. ENDCLASS. CLASS cls IMPLEMENTATION. METHOD meth. DATA field LIKE attr VALUE 'local field'. ENDMETHOD. ENDCLASS.