Lecture on &Option<T> and Option<&T>

Introduction

• Two Rust types: &Option<T> and Option<&T>
• Argument: never use &Option<T>, always use Option<&T>
• Explanation of why Option<&T> is preferred

Example Struct: Data

• Large struct, doesn't implement Copy or Clone
• Method: crunch -> returns an integer
• Example Struct: Widget
  • Field: Option<Data>
  • Two accessor methods: data_a and data_b

Two Accessor Methods

• data_a: returns a &Option<Data>
• data_b: returns an Option<&Data>
• Uses as_ref to convert between types

Analysis of Function Signatures

• &Option<Data>: reference to the option, allows changes to option state
• Option<&Data>: reference to data if it exists, no changes to option state
• Mutability changes provide clearer differentiation
  • &mut Option<Data> vs Option<&mut Data>
  • Mutating option vs mutating data only

Advantages of Option<&T>

1. Convenience: Call sites don't need to call as_ref themselves
2. Encapsulation: Implementation details can change without API changes

• Modifying internal storage without affecting API
  • Example: changing Option<Data> to Option<Box<Data>>

3. Flexibility: Abstracting over various data sources

Practical Examples

• Calling methods on struct Widget
  • data_a: must use as_ref for further operations
  • data_b: works directly, easier to use

Option Types in Parameters

• &Option<T> vs Option<&T> for function params
  • Forces data to be in an Option when using &Option<T>
  • Flexibility with Option<&T>

Memory Layout

• Option types and memory alignment
• Niche optimization: Option<&T> uses null pointer optimization

Conclusion

• Recommendation: avoid &Option<T> in APIs
• Use Option<&T> for flexibility, encapsulation, and ease of use

Final Note

• Invites valid use cases for &Option<T> in comments.