如何使用copy-on-write语义创建容器？(迅速)

Question

我有一个非常大的结构,我想确保不会被不必要地复制.如何为它创建一个copy-on-write容器？

Answer 1

写时复制通常是struct某些后备对象的包装器.

public final class MutableHeapStore<T>: NonObjectiveCBase
{
    public typealias Storage = T

    public private(set) var storage: Storage

    public init(storage: Storage)
    {
        self.storage = storage
    }
}

public struct COW<T>
{
    public typealias Storage = MutableHeapStore<T>
    public typealias Value = T

    public var storage: Storage

    public init(storage: Storage)
    {
        self.storage = storage
    }

    public var value: Value
    {
        get
        {
            return storage.storage
        }

        set
        {
            if isUniquelyReferenced(&storage)
            {
                storage.storage = newValue
            }

            else
            {
                storage = Storage(storage: newValue)
            }
        }
    }

    public init(_ value: Value)
    {
        self.init(storage: Storage(storage: value))
    }
}

extension COW: CustomStringConvertible
{
    public var description: String
    {
        return String(value)
    }
}

诀窍在于isUniquelyReferenced每次盒装值发生变异时断言.如果单独引用底层存储对象,则不执行任何操作.但是,如果存在另一个引用,则必须创建新存储.

这段代码是线程安全的吗？它与任何其他值类型一样安全,例如Int或Bool.