Swift 通用工厂:错误?

Swift Generic Factory: Bug?

调查 Swift 泛型,并偶然发现一些非常奇怪的行为...我应该提交雷达,还是我误解了什么?使用 Swift 1.2 beta 测试。

代码说话最好,简单的实现限制了工厂实例化 BaseClass 和派生 类:

/// Testing factory pattern to instantiate BaseClass and derived
class BaseClassFactory
{
    // Always returns an instance of BaseClass, even if the constant or
    // variable storing the result is explicitly typed( see test )
    class func makeInstance< T : BaseClass >( type: AnyClass ) -> T?
    {
        return T()
    }

    // Returns an instance of DerivedClass only if the constant or variable storing 
    // the result is explicitly typed.
    class func makeInstanceContrived< T : BaseClass >( type: AnyClass ) -> T?
    {
        let instance : T? = T.makeInstance() as? T
        return instance
    }

    // Works fine
    class func makeInstanceAndCallBack< T : BaseClass >( handler: ( instance: T? ) -> Void ) -> Void
    {
        let myInstance : T? = T.makeInstance() as? T

        handler( instance: myInstance )
    }
}

class BaseClass
{
    // Since T() fails... 
    class func makeInstance()->BaseClass
    {
        return BaseClass()
    }
}

class DerivedClass : BaseClass
{
    override class func makeInstance()->BaseClass
    {
        return DerivedClass()
    }
}

测试,带有非常奇怪行为的屏幕截图(尽管有编译器警告,'is' 测试确实通过了):

// Nope
if let instance = BaseClassFactory.makeInstance( DerivedClass.Type )
{
    if instance is DerivedClass == false
    {
        println( "1: Wrong type..." )
    }
}

// Nope, even when typing the constant. This seems like very dangerous behaviour...
if let instance : DerivedClass = BaseClassFactory.makeInstance( DerivedClass.Type )
{
    if instance is DerivedClass == false
    {
        //compiler even gives a warning here: " 'is' test is always true "
        println( "2: what the???" )
    }
}

// Nope
if let contrivedInstance = BaseClassFactory.makeInstanceContrived( DerivedClass.Type )
{
    if contrivedInstance is DerivedClass == false
    {
        println( "3: Wrong type..." )
    }
}

// Yes, typing the constant does the trick here
if let contrivedInstance : DerivedClass = BaseClassFactory.makeInstanceContrived( DerivedClass.Type )
{
    println( "4: success! type is: \(contrivedInstance )" )
}

// Yes
BaseClassFactory.makeInstanceAndCallBack()
{
    ( instance: DerivedClass? ) -> Void in

    if let callbackInstance = instance
    {
        println( "5: success! type is: \(callbackInstance )" )
    }
}

你在这里有两个问题——一个是不正确的代码,另一个是一个已知的错误(我的雷达作为 18518629 的复制品被关闭,它在 1.2b4 中仍然是开放的)。

首先,在这个构造中:

class func makeInstance< T : BaseClass >( type: AnyClass ) -> T?
{
    return T()
}

// then later

BaseClassFactory.makeInstance( DerivedClass.Type )

你的论点没有任何作用。它本质上是毫无意义的,对 T 的类型没有贡献(怎么可能?参数没有引用 T)。相反,T 的类型将从上下文中选择,即如果您将结果分配给 DerivedClass 变量,T 将是 DerivedClass。如果您不指定,默认行为是将 T 作为基数 class,它被限制为 BaseClass

你的意思大概是这样的:

class func makeInstance< T : BaseClass >( type: T.Type ) -> T?
{
    return T()
}

// then later

BaseClassFactory.makeInstance( DerivedClass.self )

这应该可以将 T 设置为您想要的类型。但它仍然无法工作,因为基 class 没有动态调度的初始化程序导致的错误(泛型只有一个运行时实现,它依赖于正确的 init 被多态调用) .

如果您将 required init() { } 添加到 BaseType,您将获得正确的行为。