Best approach to determining the memory layout of SwiftProtobuf message types with Storage classes?

[MrMage]

I am trying to determine the allocated size of my generated SwiftProtobuf message types in order to see whether I could e.g. reduce their memory usage by changing the field order of smaller types inside them.

However, for types without inline storage, the _StorageClass types are fileprivate, which means that I can't easily measure their size with class_getInstanceSize.

Right now I am using the following workaround based on reflection:

	private func getMemoryUsageOfStorageClass(of messageType: Message.Type) -> Int? {
		let mirror = Mirror(reflecting: messageType.init())
		for case let (label?, value) in mirror.children {
			if label == "_storage" {
				return class_getInstanceSize(type(of: value) as? AnyClass)
			}
		}
		return nil
	}

Is there a better workaround for this, or should I just keep using that?

[thomasvl]

I'm not aware of anything better than that. But the decisions around internal layout for size is something that's currently counted as an implementation detail and subject to change. We've had changes in this space in the past, and there are open issues about it (#735, #733). Other protobuf libraries do tweak the ordering to get better layout, it isn't something we've gone after yet for Swift, but is likely something that might eventually get some attention.

[MrMage]

Thank you for the elaboration. For now, I'll at least add tests for my messages' current sizes in order to spot any regressions.