Swift: How to use sizeof?

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Updated for Swift 3

Be careful that MemoryLayout<T>.size means something different than sizeof in C/Obj-C. You can read this old thread https://devforums.apple.com/message/1086617#1086617

Swift uses an generic type to make it explicit that the number is known at compile time.

To summarize, MemoryLayout<Type>.size is the space required for a single instance while MemoryLayout<Type>.stride is the distance between successive elements in a contiguous array. MemoryLayout<Type>.stride in Swift is the same as sizeof(type) in C/Obj-C.

To give a more concrete example:

struct Foo {
  let x: Int
  let y: Bool
}

MemoryLayout<Int>.size      // returns 8 on 64-bit
MemoryLayout<Bool>.size     // returns 1
MemoryLayout<Foo>.size      // returns 9
MemoryLayout<Foo>.stride    // returns 16 because of alignment requirements
MemoryLayout<Foo>.alignment // returns 8, addresses must be multiples of 8

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