Dictionary uses two storage schemes: native storage and Cocoa storage.

只看 native storage 的，也就是和 OC 无关的。

类图



内存分布



分配内存的地方：

  static internal func allocate(
    scale: Int8,
    age: Int32?,
    seed: Int?
  ) -> _DictionaryStorage {
    // The entry count must be representable by an Int value; hence the scale's
    // peculiar upper bound.
    _internalInvariant(scale >= 0 && scale < Int.bitWidth - 1)

    let bucketCount = (1 as Int) &<< scale
    let wordCount = _UnsafeBitset.wordCount(forCapacity: bucketCount)
    let storage = Builtin.allocWithTailElems_3(
      _DictionaryStorage<Key, Value>.self,
      wordCount._builtinWordValue, _HashTable.Word.self,
      bucketCount._builtinWordValue, Key.self,
      bucketCount._builtinWordValue, Value.self)

    let metadataAddr = Builtin.projectTailElems(storage, _HashTable.Word.self)
    let keysAddr = Builtin.getTailAddr_Word(
      metadataAddr, wordCount._builtinWordValue, _HashTable.Word.self,
      Key.self)
    let valuesAddr = Builtin.getTailAddr_Word(
      keysAddr, bucketCount._builtinWordValue, Key.self,
      Value.self)
    storage._count = 0
    storage._capacity = _HashTable.capacity(forScale: scale)
    storage._scale = scale
    storage._reservedScale = 0
    storage._extra = 0

    if let age = age {
      storage._age = age
    } else {
      // The default mutation count is simply a scrambled version of the storage
      // address.
      storage._age = Int32(
        truncatingIfNeeded: ObjectIdentifier(storage).hashValue)
    }

    storage._seed = seed ?? _HashTable.hashSeed(for: storage, scale: scale)
    storage._rawKeys = UnsafeMutableRawPointer(keysAddr)
    storage._rawValues = UnsafeMutableRawPointer(valuesAddr)

    // Initialize hash table metadata.
    storage._hashTable.clear()
    return storage
  }

和 Set 区别

区别仅仅在于storage尾部有两个数组，一个存储key，一个存储value。
进行各种操作时，要同时操作key和value。

  @inlinable // FIXME(inline-always) was usableFromInline
  @inline(__always)
  internal func uncheckedInitialize(
    at bucket: Bucket,
    toKey key: __owned Key,
    value: __owned Value) {
    defer { _fixLifetime(self) }
    _internalInvariant(hashTable.isValid(bucket))
    (_keys + bucket.offset).initialize(to: key)
    (_values + bucket.offset).initialize(to: value)
  }