Get the size (in bytes) of an object on the heap

Question

I'm aware you can use MemoryLayout<T>.size to get the size of a type T.

For example: MemoryLayout<Int32>.size // 4

However, for class instances (objects), MemoryLayout<T>.size returns the size of the reference to the object (8 bytes on 64 bit machines), not the size of the actual objects on the heap.

class ClassA { // Objects should be at least 8 bytes
    let x: Int64 = 0
}

class ClassB  {// Objects should be at least 16 bytes
    let x: Int64 = 0
    let y: Int64 = 0
}

MemoryLayout<ClassA>.size // 8
MemoryLayout<ClassB>.size // 8, as well :(

How can I get the size of the objects themselves?

For those wondering, I have no real need for this, I'm just exploring around Swift and its interoperability with C.

Answer 1

One option on Apple platforms, because Swift classes are currently built on top of Objective-C classes there, would be to use the Obj-C runtime function class_getInstanceSize, which gives you the size in bytes of an instance of the class, including any padding.

// on a 64-bit machine (1 word == 8 bytes)...

import Foundation

class C {}
print(class_getInstanceSize(C.self)) // 16 bytes metadata for empty class 
                                     // (isa ptr + ref count)

class C1 {
    var i = 0
    var i1 = 0
    var b = false
}

print(class_getInstanceSize(C1.self)) // 40 bytes
// (16 metadata + 24 ivars, 8 for i + 8 for i1 + 1 for b + 7 padding)