EXPRESS Overview

The data models for STEP are defined using EXPRESS (ISO 10303-11), an information modeling language.

1. What is EXPRESS?

• A data definition language with object-oriented-like characteristics.
• It defines Entities, Types, Functions, and Rules.
• A STEP Application Protocol (e.g., AP214) is essentially a massive schema written in EXPRESS.

Relationship: Schema vs. Data

2. How to Read the Schema

ENTITY product;
  id : identifier;
  name : label;
  description : OPTIONAL text;
  frame_of_reference : SET [1:?] OF product_context;
END_ENTITY;

• ENTITY: Similar to a class.
• id, name: Attributes.
• OPTIONAL: The data value may be null.
• SET [1:?]: A set of one or more unique items.

Entity Inheritance (SUPERTYPE/SUBTYPE)

EXPRESS supports inheritance through SUPERTYPE and SUBTYPE relationships. This allows entities to inherit attributes from parent entities:

Key Concepts:

• SUPERTYPE: Parent entity (abstract, cannot be instantiated directly)
• SUBTYPE: Child entity (inherits all attributes from parent)
• Abstract Entities: Cannot appear in Part 21 files directly (e.g., GEOMETRIC_REPRESENTATION_ITEM)
• Concrete Entities: Can be instantiated in Part 21 files (e.g., ADVANCED_FACE, EDGE_CURVE)

Example EXPRESS Definition:

ENTITY geometric_representation_item
  ABSTRACT SUPERTYPE OF (ONEOF (advanced_face, edge_curve, vertex_point));
  -- No attributes, just a base type
END_ENTITY;

ENTITY advanced_face
  SUBTYPE OF (geometric_representation_item);
  bounds : SET [1:?] OF face_bound;
  face_geometry : surface;
  same_sense : BOOLEAN;
END_ENTITY;

What This Means for Implementers:

• When parsing, you may encounter references to abstract types, but actual instances will always be concrete subtypes.
• Type checking should consider inheritance: an ADVANCED_FACE is also a GEOMETRIC_REPRESENTATION_ITEM.

3. What Implementers Need to Know

During implementation, rules like "which attributes are mandatory" and "which types are convertible" are all based on these EXPRESS definitions found in the standard.

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📚 Next Steps

• Common Pitfalls - Problems often encountered during implementation and their solutions.

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