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The entity is a fundamental concept in
ACIS, and the
ENTITY class is the fundamental data structure in
ACIS that supports roll back, copy, save/restore and error recovery. All persistent data types in an
ACIS model are derived from
ENTITY or are directly controlled by an
ENTITY derived class. Derived
ENTITYs make up the foundation of
ACIS. Derivation of new
ENTITY classes is also a common means of extending
ACIS, and application developers can derive classes from
ENTITY to define their own persistent data types. Attributes are special entities containing user-defined data that can be attached to other entities.
ATTRIB is a derived class of
ENTITY and is the base class for implementing attributes.
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In exchange for the power of classes derived from
ENTITY, the application programmer bears the burden of properly implementing a large number of functions (virtual functions of
ENTITY). Fortunately
ACIS provides a number of macros (defined in
entity.hxx), that ease this burden considerably. There are macros to fully or partially implement each of the required virtual functions. Macros specific to attributes are defined in
at_macro.hxx.
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The use of these macros also protects the programmer from possible changes to the function signatures in different versions of
ACIS. For example, in
ACIS Release 5.0, a new argument was added to the
copy_scan function. Only code that actually uses the type information needed to be modified. The majority of derived
ENTITYs could simply be recompiled with the revised header file.
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This chapter describes the macros used when deriving classes from
ENTITY (or
ATTRIB), the functions generated by the macros, when those functions are called, and how the developer should implement the code specific to his own
ENTITY derivations. Attribute derivation is discussed in more detail in the attributes overview chapter of the
Kernel Component Manual.
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