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[cxx-interop] Allow initializing std::map from Swift Dictionary #75877

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Merged
merged 1 commit into from
Aug 14, 2024
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[cxx-interop] Allow initializing std::map from Swift Dictionary #75877

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14 changes: 14 additions & 0 deletions stdlib/public/Cxx/CxxDictionary.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,20 @@ public protocol CxxDictionary<Key, Value> {
}

extension CxxDictionary {
/// Creates a C++ map containing the elements of a Swift Dictionary.
///
/// This initializes the map by copying every key and value of the dictionary.
///
/// - Complexity: O(*n*), where *n* is the number of entries in the Swift
/// dictionary
@inlinable
public init(_ dictionary: Dictionary<Key, Value>) where Key: Hashable {
self.init()
for (key, value) in dictionary {
self[key] = value
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As a future performance optimization, we could use __insertUnsafe here to skip the check for the presence of the element in subscript.

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Any reason not to do it in this PR? Are there any concerns about that approach?

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No strong reasons, but I wanted to add a simple benchmark first (in a different PR) to see if the unsafe APIs actually give us any speedup compared to the subscript.

}
}

@inlinable
public subscript(key: Key) -> Value? {
get {
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43 changes: 43 additions & 0 deletions test/Interop/Cxx/stdlib/use-std-map.swift
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Expand Up @@ -28,6 +28,49 @@ StdMapTestSuite.test("init") {
expectTrue(m.empty())
}

StdMapTestSuite.test("Map.init(_: Dictionary<Int, Int>)") {
let swiftDict: [Int32 : Int32] = [-1: 2, 2: 3, 33: 44]
let m = Map(swiftDict)
expectEqual(m.size(), 3)

expectEqual(m[-1], 2)
expectEqual(m[2], 3)
expectEqual(m[33], 44)

let emptySwiftDict: [Int32 : Int32] = [:]
let emptyM = Map(emptySwiftDict)
expectEqual(emptyM.size(), 0)
}

/// Same as above, but for std::unordered_map.
StdMapTestSuite.test("UnorderedMap.init(_: Dictionary<Int, Int>)") {
let swiftDict: [Int32 : Int32] = [-1 : 2, 2 : 3, 33 : 44]
let m = UnorderedMap(swiftDict)
expectEqual(m.size(), 3)

expectEqual(m[-1], 2)
expectEqual(m[2], 3)
expectEqual(m[33], 44)

let emptySwiftDict: [Int32 : Int32] = [:]
let emptyM = UnorderedMap(emptySwiftDict)
expectEqual(emptyM.size(), 0)
}

StdMapTestSuite.test("MapStrings.init(_: Dictionary<std.string, std.string>)") {
let swiftDict = [std.string("abc") : std.string("123"),
std.string() : std.string("empty")]
let m = MapStrings(swiftDict)
expectEqual(m.size(), 2)

expectEqual(m[std.string("abc")], std.string("123"))
expectEqual(m[std.string()], std.string("empty"))

let emptySwiftDict: [std.string : std.string] = [:]
let emptyM = MapStrings(emptySwiftDict)
expectEqual(emptyM.size(), 0)
}

StdMapTestSuite.test("Map.subscript") {
// This relies on the `std::map` conformance to `CxxDictionary` protocol.
var m = initMap()
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