类 java 继承的 VBA 组合

Question

在这个问题上稍微扩展一下 VBA inheritance pattern

我正在用 VBA 重现一个基本的继承模式，但我想了解是否有更有效、更简洁的方法来实现这一目标。

考虑这个小测试用例。

IAnimal.cls

'declaration
Public Sub eat()
End Sub

Public Sub breathe()
End Sub

Animal.cls:父类(super class)

Implements IAnimal

' method implementation
Private Sub IAnimal_eat()
    Debug.Print "I'm eating something..."
End Sub

Private Sub IAnimal_breathe()
    Debug.Print "I'm brething..."
End Sub

Cat.cls : Animal 的一个子类

Private super As IAnimal

Implements IAnimal

Private Sub Class_Initialize()
    Set super = New Animal
End Sub


'#------------------------------------------------
' methods called when object is accessed as an IAnimal implementor. 
' I HAVE TO re-implement all of them also here in the subclass (in java I don't need to. It's sufficient to implement them in the superclass)
Private Sub IAnimal_eat()
    Me.eat
End Sub

Private Sub IAnimal_breathe()
    Me.breathe
End Sub


'#--------------------------------------------------
' subclass-only methods
' To access those methods I MUST DECLARE the object as Cat (Dim tom as Cat)
Public Sub meow()
Debug.Print "meow..."
End Sub


'#------------------------------------------------ 
' superclass methods
' Since I need to declare the cat object as a Cat (see above)
' I'm FORCED TO explicitly re-implement all of the superclass methods,
' even those that I don't need to override
' otherwise I can't access them

'@Override
Public Sub eat()
    Debug.print "I'm  eating a fish!"
End Sub

'I'm forced to re-implement also this method, in order to use it directly on a *Cat* object
'@Unnecessary-Override
Public Sub breathe()
    super.breathe 
End Sub

测试.bas

Sub Main()

    Dim snowball As IAnimal
    Set snowball = New Cat

    Dim tom As Cat
    Set tom = New Cat

    snowball.meow  'ERROR Method or data member not found <---- cannot access the Cat-only method "meow"
    tom.meow  '<--- prints "meow"

    ' creates a Dog, another subclass of Animal
    Dim pluto As Dog
    Set pluto = New Dog

    'create a collection for all my animals
    Dim myPets As Collection
    Set myPets = New Collection

    myPets.Add tom
    myPets.Add pluto

    Call feed(myPets) '<---- prints
                            'I 'm eating a fish
                            'meow...
                            'I 'm eating a bone
                            'woof...

End Sub

' a routine for showing how to manage an heterogeneous collection of animals
Sub feed(animals As Collection)

    For Each a In animals

        a.eat

        If TypeOf a Is Cat Then
            a.meow
        End If

        If TypeOf a Is Dog Then
            a.woof
        End If

    Next

End Sub

通过这样做，我可以:

• 对 Cat 和 Dog 对象调用 eat() 方法并具有特定的重写行为
• 调用仅限子类的方法(如meow())
• 将一个异类动物集合传递给一个feed子例程，它可以“安全地”调用Animal父类(super class)的方法并触发基于Animal子类的特定代码<

这似乎可行但很麻烦:假设您需要实现许多 Animal 子类(Dog、Bird、Armadillo、Platypus、Demogorgon 等)。上面的模式迫使你:

1. 在所有子类上重新实现 IAnimal 接口(interface)的所有方法
2. (再次)重新实现所有方法，以从子类中公开它们，即使不需要重写。如果您还想访问仅限子类的方法，这是特别需要的。

所以问题是:是否有更高效/简洁的方法来实现这个示例(并限制每个子类的代码重写)？

Answer 1

tom 一开始就不应该声明为 As Cat； feed 过程是多余的:

Sub Main()    
    Dim tom As IAnimal
    Set tom = New Cat    
    tom.eat    
End Sub

现在，在 Cat 类中，这些成员不需要存在:

'Superclass methods --- I have to explicitly override all methods :(
Public Sub eat()
    super.eat 
    Debug.print "...a fish!"
End Sub

在 SOLID/OOP 中，您针对接口(interface)而不是具体类型进行编码 - 这就是为什么 tom 是 IAnimal 而不是 Cat .通过它的 IAnimal 接口(interface)访问，Cat.eat 是完全多余的，并且暗示 Cat 做一些 IAnimal 不这样做，这违反了 SOLID 原则:突然间它变得相关，一个 IAnimal 是一个 Cat，它不应该是，因为多态性允许 IAnimal 是任何东西，Liskov 替换原则(LSP - “SOLID”的“L”)表示任何与 IAnimal 一起工作的代码都应该能够无论给定该接口(interface)的实现方式如何，都能以相同的方式工作。

遵守这些原则意味着任何 IAnimal 实现都不应在其默认接口(interface)上具有 IAnimal 成员的副本(例如 Cat.eat， vs IAnimal.eat)，这完全消除了你的第 2 点:

2. re-implements (again) all the methods, to expose them from the subclass, even when override is not necessary.

至于第 1 点......

1. re-implement all the methods of the IAnimal interface

这是编译器的要求，而不是 VBA 的怪癖:无论是在 Java、C# 还是 VBA 中，你都不能说“我正在实现一个接口(interface)”……而不实现它的成员。当然，Java 和 C# 允许类继承，因此您的基类可以说“我正在实现一个接口(interface)”，实现所有成员，派生类会很乐意继承它们 - 但是，这就是继承，不再是组合。