A presentation of Mike Godfrey's work I gave at a reengineering course.

1. Cloning Considered Harmful
Considered Harmful?
presented by
Nicolas Bettenburg
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2. 10-15% cloned!
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3. Intentional
e.g., Copy & Paste
{
Unintentional
} e.g., Language Idioms
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4. Cloning
= additional maintenance effort
B B B B
A A A A A
1.1 1.2 1.3 1.4.0 1.4.1
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5. Clones considered
harmful!
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6. But Wait!
Is that the right thing
to do?
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7. Example: Cloning
to experiment without risk
experimental
stable
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8. Understand Reason
behind duplication
before deciding!
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9. Catalogue of
High-Level
Cloning Patterns
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10. 8 general cloning
patterns
Forking Templating Customization
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11. Hardware Variations
Clone existing driver
+ No risk of changing existing driver
- Code growth
~ Dead code can creep into system
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12. Platform Variations
Clone existing code and fix
low level platform interaction
+ Avoid complexity of virtualization layer
- Hard to propagate bug fixes
~ Ensure consistent behavior of all clones
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13. Experimental Variations
Clone existing code and experiment
+ Maintain stable code
- Merging later may be difficult
~ Consistent maintenance becomes difficult
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14. Boiler-plating
Cloning due to language constraints
+ Make reuse of trusted code possible
- Increased maintenance (close evolution)
~ Explicit and rigorous linking of all duplicates
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15. API/Library Protocols
Clone reoccurring call sequences
+ Learn from code and reduce effort
- Degradation of Code Quality
~ Changes to API require maintenance of all clones
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16. Language Idioms
a = malloc(64);
if (a == null) {
out(“Error!”);
}
...
Idioms provide standardization
+ Selfdocumenting and improve comprehensibility
- Faulty implementations are easily overlooked
~ no long term issues
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17. Bug Workarounds
... ...
a = sqroot(81); a = sqroot(81);
... if (a != 9) {
a = 9;
}
...
Clone to provide work arounds
+ Clone buggy code and fix
- Source of the bug is not addressed
~ Remove clone when bug is fixed
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18. Replicate & Specialize
Clone to reuse and adapt existing solutions
+ Less effort needed
- Long-term cost outweighs short-term benefit
~ Cost of refactoring rises over time
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19. Conclusions
•Templating requires synchronization
•Variation requires monitoring of changes
•Forking need outlining
•Not all clones require refactoring
•Some situations warrant the effort
•Measure risk vs. long-term cost
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20. Considering Cloning harmful
in general can be harmful!
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