8.21.2.2 IfcGloballyUniqueId
8.21.2.2.1 Semantic definition
An IfcGloballyUniqueId holds an encoded string identifier that is used to uniquely identify an IFC object. An IfcGloballyUniqueId is a Globally Unique Identifier (GUID) which is an auto-generated 128-bit number. Since this identifier is required for all IFC object instances, it is desirable to compress it to reduce overhead. The encoding of the base 64 character set is shown below:
1 2 3 4 5 6
0123456789012345678901234567890123456789012345678901234567890123
"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz_$";
The resulting string is a fixed 22 character length string to be exchanged within the IFC exchange file structure.
The base64 encoding process may differ from common base64 implementations. The following steps are used:
- The first byte is encoded in the first two characters
- The remaining bytes are encoded in groups of 3, taking up 4 characters
As a result, the first character must be either a 0, 1, 2, or 3.
8.21.2.2.2 Formal representation
TYPE IfcGloballyUniqueId = STRING(22) FIXED;
END_TYPE;