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Static Boxes

Static boxes are boxed components whose components are known statically and which are subject to initial value sharing. Declarations of static boxes generally require less memory for structures that occur multiple times but are rarely used.

The following can currently be declared as static boxes:

• Substructures of structured data with the addition BOXED of the statement TYPES

• Structured attributes of classes or interfaces with the addition BOXED of the statement [CLASS-]DATA.

A static box can have one of two states:

• Initial value sharing
  
  As long as none of the actions named in the following point were executed, initial value sharing applies to a static box. The internal reference points to an appropriate initial value for the structure type, which is stored exactly once in each application server in the PXA. The memory requirement in the internal session is determined only by the internal reference and its administration.

• Revoked initial value sharing
  
  The following actions revoke initial value sharing for a static box:

• Write access to the static box or one of its components

• Assigning the static box or one of its components to a field symbol using ASSIGN

• Addressing the static box or one of its components using a data reference

• Using a static box or one of its components as an actual parameter for procedure calls

The internal reference then references an instance of the structure in the current internal session. The memory requirement is the same as for a normal structure plus the administration costs for the internal reference.

In initial value sharing, the memory required for the internal session of a static box is not initially known when the program is executed. However, unlike with real dynamic data objects, the length of a static box is always known statically. In static boxes, the same functions are available as those available for a normal component of the same type. When a structure that contains a static box is accessed, however, it must not be forgotten that it is a deep component.

Other versions: 7.31 | 7.40 | 7.54

Notes

• Static boxes can be used to optimize the memory requirement of structures that are used more than once. If, for example, the row structure of an internal table contains substructures, the memory requirement of the substructure accumulates without the use of static boxes for each row, even if the substructure is initial. If static boxes are used, initial substructures do not require multiple memory stores as long as they permit read access only.

• In addition to the optimized memory requirements, static boxes generally have a positive impact on runtime performance, because assignments between components for which initial value sharing is active only require the internal reference (and no additional data) to be copied.

• The CLEAR and FREE statements do not work as write statements on a static box that has the initial value sharing state, and the state is retained. On the other hand, once the initial value sharing status is revoked, these statements do not currently free up any memory, but provide the local instance of the static box with appropriate initial values.