Designing OOP R

This skill covers object-oriented programming in R, including S7, S3, S4, and vctrs-based classes.

Decision Tree: What Are You Building?

1. Vector-like objects (behave like atomic vectors)

Use vctrs when:

• Need data frame integration (columns/rows)
• Want type-stable vector operations
• Building factor-like, date-like, or numeric-like classes
• Need consistent coercion/casting behavior
• Working with existing tidyverse infrastructure

Examples: custom date classes, units, categorical data

2. General objects (complex data structures, not vector-like)

Use S7 when:

• NEW projects that need formal classes
• Want property validation and safe property access (@)
• Need multiple dispatch (beyond S3's double dispatch)
• Converting from S3 and want better structure
• Building class hierarchies with inheritance
• Want better error messages and discoverability

Use S3 when:

• Simple classes with minimal structure needs
• Maximum compatibility and minimal dependencies
• Quick prototyping or internal classes
• Contributing to existing S3-based ecosystems
• Performance is absolutely critical (minimal overhead)

Use S4 when:

• Working in Bioconductor ecosystem
• Need complex multiple inheritance (S7 doesn't support this)
• Existing S4 codebase that works well

S7 vs S3 Comparison

Feature	S3	S7	When S7 wins
Class definition	Informal (convention)	Formal (new_class())	Need guaranteed structure
Property access	$ or attr() (unsafe)	@ (safe, validated)	Property validation matters
Validation	Manual, inconsistent	Built-in validators	Data integrity important
Method discovery	Hard to find methods	Clear method printing	Developer experience matters
Multiple dispatch	Limited (base generics)	Full multiple dispatch	Complex method dispatch needed
Inheritance	Informal, NextMethod()	Explicit super()	Predictable inheritance needed
Migration cost	-	Low (1-2 hours)	Want better structure
Performance	Fastest	~Same as S3	Performance difference negligible
Compatibility	Full S3	Full S3 + S7	Need both old and new patterns

S7: Modern OOP

S7 combines S3 simplicity with S4 structure. See s7-examples.md for:

• Class definitions with new_class()
• Property validation
• Generic and method definition
• Inheritance with parent

S3: Simple Classes

See s3-examples.md for:

• Constructor functions
• Print and format methods
• Simple class patterns

Practical Guidelines

Choose S7 when you have:

See when-s7.md for:

• Complex validation needs
• Multiple dispatch needs
• Class hierarchies with clear inheritance

Choose vctrs when you need:

See when-vctrs.md for:

• Vector-like behavior in data frames
• Type-stable operations

Choose S3 when you have:

See when-s3.md for:

• Simple classes without complex needs
• Maximum performance needs (rare)
• Existing S3 ecosystem contributions

Migration Strategy

1. S3 → S7: Usually 1-2 hours work, keeps full compatibility
2. S4 → S7: More complex, evaluate if S4 features are actually needed
3. Base R → vctrs: For vector-like classes, significant benefits
4. Combining approaches: S7 classes can use vctrs principles internally

source: Sarah Johnson's gist https://gist.github.com/sj-io/3828d64d0969f2a0f05297e59e6c15ad