Editing Object copying (section)

=== Combination ===
In more complex cases, some fields in a copy should have shared values with the original object (as in a shallow copy), corresponding to an "association" relationship; and some fields should have copies (as in a deep copy), corresponding to an "aggregation" relationship. In these cases a custom implementation of copying is generally required; this issue and solution dates to Smalltalk-80.{{sfn|Goldberg|Robson|1983|page=97 |ps=. "The default implementation of <code>copy</code> is <code>shallowCopy</code>. In subclasses in which copying must result in a special combination of shared and unshared variables, the method associated with copy is usually reimplemented, rather than the method associated with <code>shallowCopy</code> or <code>deepCopy</code>."}} Alternatively, fields can be marked as requiring a shallow copy or deep copy, and copy operations automatically generated (likewise for comparison operations).{{sfn|Grogono|Sakkinen|2000}} This is not implemented in most object-oriented languages, however, though there is partial support in Eiffel.{{sfn|Grogono|Sakkinen|2000}}