获取堆上对象的大小(以字节为单位) [英] Get the size (in bytes) of an object on the heap

问题描述

我知道您可以使用MemoryLayout<T>.size来获取类型T的大小.

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

例如:MemoryLayout<Int32>.size // 4

但是，对于类实例(对象)，MemoryLayout<T>.size返回对对象的引用的大小(在64位计算机上为8字节)，而不是堆上实际对象的大小.

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 :(

如何获取对象本身的大小?

How can I get the size of the objects themselves?

对于那些想知道的人，我并没有真正的需要，我只是在探索Swift及其与C的互操作性.

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

推荐答案

在 Apple平台上的一个选项，因为Swift类是 class_getInstanceSize ，它为您提供了该类实例的大小(以字节为单位) ，包括任何填充.

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)

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