borrowing & consuming — Parameter Ownership

Purpose: Explicit ownership modifiers for performance optimization and noncopyable type support Swift Version: Swift 5.9+ iOS Version: iOS 13+

When to Use This Skill

✅ Use this skill when:

• Large value types being passed read-only (avoid copies)
• Working with noncopyable types (~Copyable)
• Reducing ARC retain/release traffic
• Factory methods that consume builder objects
• Performance-critical code where copies show in profiling

❌ Do NOT use this skill for:

• Simple types (Int, Bool, small structs)
• Compiler optimization is sufficient (most cases)
• You're not certain about the performance impact

Quick Reference

Modifier	Ownership	Copies	Use Case
(default)	Compiler chooses	Implicit	Most cases
borrowing	Caller keeps	Explicit copy only	Read-only, large types
consuming	Caller transfers	None needed	Final use, factories
inout	Caller keeps, mutable	None	Modify in place

Key Patterns

Read-Only Large Struct

// ✅ Explicit borrow — no copy
func process(_ buffer: borrowing LargeBuffer) -> Int {
    buffer.data.count
}

Consuming Factory

struct Builder {
    consuming func build() -> Product {
        Product(config: config)
    }
}

let product = builder.build()
// builder is now invalid

Noncopyable Type

struct FileHandle: ~Copyable {
    borrowing func read(count: Int) -> Data { /* ... */ }
    consuming func close() { /* ... */ }
}

Related Skills

• Swift Performance — Value type optimization
• Swift Concurrency — Sendable types

Resources

• Swift Evolution: SE-0377: borrowing and consuming
• WWDC: Consume noncopyable types in Swift (10170)