borrowing & consuming — Parameter Ownership

Explicit ownership modifiers for performance optimization and noncopyable type support.

When to Use

✅ Use 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

❌ Don't use when:

• Simple types (Int, Bool, small structs)

• Compiler optimization is sufficient (most cases)

• Readability matters more than micro-optimization

• 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

Default Behavior by Context

Context Default Reason

Function parameters borrowing

Most params are read-only

Initializer parameters consuming

Usually stored in properties

Property setters consuming

Value is stored

Method self

borrowing

Methods read self

Patterns

Pattern 1: Read-Only Large Struct

struct LargeBuffer { var data: [UInt8] // Could be megabytes }

// ❌ Default may copy func process(_ buffer: LargeBuffer) -> Int { buffer.data.count }

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

Pattern 2: Consuming Factory

struct Builder { var config: Configuration

// Consumes self — builder invalid after call
consuming func build() -> Product {
    Product(config: config)
}

}

let builder = Builder(config: .default) let product = builder.build() // builder is now invalid — compiler error if used

Pattern 3: Explicit Copy in Borrowing

With borrowing , copies must be explicit:

func store(_ value: borrowing LargeValue) { // ❌ Error: Cannot implicitly copy borrowing parameter self.cached = value

// ✅ Explicit copy
self.cached = copy value

}

Pattern 4: Consume Operator

Transfer ownership explicitly:

let data = loadLargeData() process(consume data) // data is now invalid — compiler prevents use

Pattern 5: Noncopyable Type

For ~Copyable types, ownership modifiers are required:

struct FileHandle: ~Copyable { private let fd: Int32

init(path: String) throws {
    fd = open(path, O_RDONLY)
    guard fd >= 0 else { throw POSIXError.errno }
}

borrowing func read(count: Int) -> Data {
    // Read without consuming handle
    var buffer = [UInt8](repeating: 0, count: count)
    _ = Darwin.read(fd, &buffer, count)
    return Data(buffer)
}

consuming func close() {
    Darwin.close(fd)
    // Handle consumed — can't use after close()
}

deinit {
    Darwin.close(fd)
}

}

// Usage let file = try FileHandle(path: "/tmp/data.txt") let data = file.read(count: 1024) // borrowing file.close() // consuming — file invalidated

Pattern 6: Reducing ARC Traffic

class ExpensiveObject { /* ... */ }

// ❌ Default: May retain/release func inspect(_ obj: ExpensiveObject) -> String { obj.description }

// ✅ Borrowing: No ARC traffic func inspect(_ obj: borrowing ExpensiveObject) -> String { obj.description }

Pattern 7: Consuming Method on Self

struct Transaction { var amount: Decimal var recipient: String

// After commit, transaction is consumed
consuming func commit() async throws {
    try await sendToServer(self)
    // self consumed — can't modify or reuse
}

}

Common Mistakes

Mistake 1: Over-Optimizing Small Types

// ❌ Unnecessary — Int is trivially copyable func add(_ a: borrowing Int, _ b: borrowing Int) -> Int { a + b }

// ✅ Let compiler optimize func add(_ a: Int, _ b: Int) -> Int { a + b }

Mistake 2: Forgetting Explicit Copy

func cache(_ value: borrowing LargeValue) { // ❌ Compile error self.values.append(value)

// ✅ Explicit copy required
self.values.append(copy value)

}

Mistake 3: Consuming When Borrowing Suffices

// ❌ Consumes unnecessarily — caller loses access func validate(_ data: consuming Data) -> Bool { data.count > 0 }

// ✅ Borrow for read-only func validate(_ data: borrowing Data) -> Bool { data.count > 0 }

Performance Considerations

When Ownership Modifiers Help

• Large structs (arrays, dictionaries, custom value types)

• High-frequency function calls in tight loops

• Reference types where ARC traffic is measurable

• Noncopyable types (required, not optional)

When to Skip

• Default behavior is almost always optimal

• Small value types (primitives, small structs)

• Code where profiling shows no benefit

• API stability concerns (modifiers affect ABI)

Decision Tree

Need explicit ownership? ├─ Working with ~Copyable type? │ └─ Yes → Required (borrowing/consuming) ├─ Large value type passed frequently? │ ├─ Read-only? → borrowing │ └─ Final use? → consuming ├─ ARC traffic visible in profiler? │ ├─ Read-only? → borrowing │ └─ Transferring ownership? → consuming └─ Otherwise → Let compiler choose

Resources

Swift Evolution: SE-0377

WWDC: 2024-10170

Skills: axiom-swift-performance, axiom-swift-concurrency