---
title: "SE-0093: Adding a public `base` property to slices"
framework: swift-evolution
role: article
path: swift-evolution/0093-slice-base
---

# SE-0093: Adding a public `base` property to slices

* Proposal: [SE-0093](0093-slice-base.md)
* Author: [Max Moiseev](https://github.com/moiseev)
* Review Manager: [Dave Abrahams](https://github.com/dabrahams)
* Status: **Implemented (Swift 3.0)**
* Decision Notes: [Rationale](https://forums.swift.org/t/review-se-0093-adding-a-public-base-property-to-slices/2695/4)
* Implementation: [apple/swift#2929](https://github.com/apple/swift/pull/2929)

## Introduction

Slice types [provided by the standard library](https://github.com/apple/swift/blob/master/stdlib/public/core/Slice.swift.gyb) should allow public readonly access to their base collections to make efficient implementations of protocol requirements possible in conforming types.

## Motivation

The `MutableCollection` protocol conformance requires providing an implementation of the following subscript:

```swift subscript(bounds: Range<Index>) -> SubSequence { get set } ```

If the collection chooses to use one of a variety of slice types from the standard library as its `SubSequence`, the default implementation of a setter for this subscript will use the algorithm provided by the [`_writeBackMutableSlice`](https://github.com/apple/swift/blob/master/stdlib/public/core/WriteBackMutableSlice.swift) function. This approach is fine for forward collections. It is quite possible, however, that the most efficient implementation of this setter would be to simply call the `memcpy` function. Unfortunately, slice API does not provide any way to reach to the underlying base collection, even though reference to it is stored in an internal property.

## Proposed solution

We propose to export a public readonly property `base`, that will enable optimizations mentioned above. Here is how `MutableRandomAccessSlice` definition would look like:

```swift public struct MutableRandomAccessSlice<   Base : protocol<RandomAccessIndexable, MutableIndexable> > : RandomAccessCollection, MutableCollection {

/// The underlying collection of the slice   public var base: Base { get } } ```

The same change is applicable to both mutable and immutable slice types.

## Impact on existing code

The proposed change is purely additive and does not affect existing code.

## Alternatives considered

An alternative for the immutable slices would be to simply rename the already read-only `_base` property to `base` and make it public, but this way the change is not purely additive and might cause some damage inside the standard library code.