I was watching WWDC video and can not understand when they talk about

Even though all of these properties are value types, internally, they contain a class which is used to manage the lifetime of their internal data.

Can some one share me his understanding on the above point.

  • Strings, arrays and dictionaries are all value types in Swift? #mind-blown Dec 11, 2017 at 4:12
  • @RobertHarvey Yeah, there are no distinction like "primatives" vs "objects" like with Java. All types are either value types (Struct, Enums, Tuples) or reference types (Classes). Many of the value types (Strings, arrays and dictionaries) implement Copy-on-Write semantics, so they're cheap to pass around if you're not aliasing and mutating them
    – Alexander
    Dec 11, 2017 at 5:06

1 Answer 1


String, Array, and Dictionary are all structs (that get stored on the stack), but their dynamically sized "content" is stored in a buffer allocated on the stack. This buffer is part of an object, which is referenced by the struct.

Take Array for example, whose source code is available on GitHub (as with the rest of the Swift programming language). You can see that the Array struct contains a single member: internal var _buffer: _Buffer. For Array, this _Buffer type is a typealias for either _ArrayBuffer<Element> (if the Objective C runtime is being used, for faster bridging to/from NSArray), or _ContiguousArrayBuffer<Element>.

Having the _Buffer object wrapped up in a struct (Array) like this is what allows for Array to have copy-on-write behavior. Copies of the Array can be made very cheaply by simply copying the _buffer reference from the source array to the destination array. When a mutable operation is done, Array checks to see if its _buffer is uniquely referenced (that is, no other Array shares the same _buffer`).

  • If it is uniquely referenced, then the mutable operation can be done directly on the _buffer, without the possibility of unintentionally modifying another Array (since there is no other Array sharing this _buffer.)
  • If it is not uniquely referenced, then a copy is made, and the mutable operation is performed on the copy.

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