InlineArray in Swift - Memory Efficient Fixed-Size Arrays

Swift 6.2 introduces InlineArray, a new low-level data structure designed to improve performance and memory efficiency in specific scenarios. While most developers will continue using Array in everyday code, InlineArray fills an important gap for performance-critical paths where allocation cost, cache locality, and predictable memory layout matter.

In this article, we’ll explore what InlineArray is, why Apple added it, how it works internally, and when you should (and should not) use it. We’ll also walk through practical examples to understand its behavior and limitations.

Why Was InlineArray Introduced?

Swift’s standard Array is powerful and flexible, but that flexibility comes with overhead:

• Heap allocation for storage
• Reference counting
• Capacity growth logic
• Indirection when accessing elements

In performance-sensitive systems—such as SwiftUI internals, compiler tooling, numeric code, or tight loops—this overhead can become measurable.

Apple introduced InlineArray to solve a very specific problem:

Store a small, fixed number of elements directly inline, without heap allocation.

This design enables:

• Better cache locality
• Fewer allocations
• More predictable performance
• Lower memory overhead

What Is InlineArray?

InlineArray is a fixed-size, value-type collection whose elements are stored inline within the value itself, rather than in heap-allocated storage.

Conceptually:

• Array<T> - dynamically sized, heap-allocated
• InlineArray<T, N> - fixed size N, stack-like inline storage
• Once created, the size of an InlineArray cannot change.

Declaring an InlineArray

An InlineArray is defined with:

• An element type
• A compile-time constant size

let numbers: InlineArray<Int, 4> = [1, 2, 3, 4]

Key points:

• The size (4) is part of the type
• You must provide exactly that many elements
• No resizing is allowed

Accessing Elements

Element access is similar to Array:

let values: InlineArray<Int, 3> = [10, 20, 30]

print(values[0]) // 10
print(values[1]) // 20

Indexing is bounds-checked, just like Array, ensuring safety.

Mutating an InlineArray

InlineArray is a value type, so mutation works as expected:

var scores: InlineArray<Int, 3> = [80, 90, 100]

scores[1] = 95

However, you cannot append or remove elements:

// ❌ Not allowed
scores.append(110)

The size is fixed at compile time.

InlineArray vs Array

Let’s compare the two side by side:

Memory Layout and Performance Benefits

The biggest advantage of InlineArray is memory layout predictability.

Because elements are stored inline:

• The entire collection lives contiguously
• No pointer chasing is required
• CPU cache usage improves

This is especially beneficial in:

• Math / geometry types
• Small buffers
• Internal framework data structures
• Hot paths in performance-critical code

Example: Representing a 3D Vector

Instead of:

struct Vector3 {
    var values: [Float]
}

You can now write:

struct Vector3 {
    var values: InlineArray<Float, 3>
}

let position = Vector3(values: [1.0, 2.0, 3.0])

Benefits:

• No heap allocation
• Better cache locality
• Clear semantic intent (exactly 3 values)

Iterating Over InlineArray

You can iterate using standard loops:

let colors: InlineArray<String, 3> = ["Red", "Green", "Blue"]

for color in colors {
    print(color)
}

This feels familiar while preserving the fixed-size guarantees.

Interoperability with Array

You can convert an InlineArray to a standard Array when needed:

let inline: InlineArray<Int, 4> = [1, 2, 3, 4]
let array = Array(inline)

This is useful when interacting with APIs that expect [T].

When Should You Use InlineArray?

InlineArray is not a replacement for Array. It shines in narrow, well-defined cases.

Good Use Cases

• Small, fixed-size collections
• Performance-critical code paths
• Math and geometry types
• Internal framework or library implementations

Avoid Using InlineArray When

• Collection size is dynamic
• API ergonomics matter more than performance
• You rely heavily on Array functions like append(), filter(), or map()

API Design Philosophy

Apple intentionally kept InlineArray minimal:

• No resizing
• Limited mutation
• Strong compile-time guarantees

This makes misuse harder and intent clearer—an important principle in Swift’s evolution.

Final Thoughts

InlineArray is a powerful addition to Swift, but it’s a specialized tool, not a general-purpose collection. For most application-level code, Array remains the best choice. However, when you need precise control over memory layout and performance, InlineArray provides a safe, expressive alternative without sacrificing Swift’s core principles.

If you’re building frameworks, performance-sensitive components, or low-level abstractions, InlineArray is well worth understanding and adopting thoughtfully.

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