初学VB.Net ,总结一下ByVal和ByRef
1 通过ByVal传递的变量,VB.Net会复制与源值相等的一个新的变量。而ByRef则相当于引用。
例如我们学习C的时候得Swap()函数
Imports System 'Test that Can't Swap A and B Class MyApp Public Shared Sub Main() Dim A As Integer = 0 Dim B As Integer = 1 Console.writeline("Source: A" & A & "B"& B) FakeSwap(A,B) ' After this FakeSwap(A,B), A still is 0 and B still is 1 Console.writeline("After fakeSwap: A" & A & "B"& B) Swap(A,B) ' After this Swap(A,B), A is 1 and B is 0 Console.writeline("After Swap: A" & A & "B"& B) End Sub
' Fake Swap Function:FakeSwap() Shared Sub FakeSwap(ByVal InA As Integer, ByVal InB As Integer) Dim Tmp As Integer Tmp = InA InA = InB InB = Tmp End Sub ' Real Swap Function :Swap() Shared Sub Swap(ByRef InA As Integer, ByRef InB As Integer) Dim Tmp As Integer Tmp = InA InA = InB InB = Tmp End Sub End Class
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2 注意的是: 如果ByVal传递的是自定义的类的一个实例,被复制的只是该实例的引用,引用所指向的资源并没有被复制。--相当于C++中的浅拷贝。
Imports System
' 类A的实例MyA作为函数 TestA(ByVal InA As A)的参数,结果应该是 ' --按值传递为浅拷贝,只是复制了一份引用--A的实例MyA和 InA共享一个资源 Class MyApp Public Shared Sub Main() Dim MyA As A Console.writeline("The Original Resource of MyA is: " & MyA.Resource) ' Call TestA() TestA(MyA) Console.writeline("After call the ByVal fun , the resource of MyA is: " & MyA.Resource) End Sub ' 函数TestA() 将MyA按值传递进去为InA 修改InA的Resource ,实际上修改的也是MyA的Resource Shared Sub TestA(ByVal InA As A) InA.Resource = 1 End Sub End Class
' 类A 有资源 resource (integer) Class A Public Resource As Integer = 0 End Class |
3 如果想实现类的实例(不是引用)的“按值“传递(深拷贝),则必须Overridde Clone()方法 ?还是专门有拷贝构造函数?
方法一:
<Serializable>_
Class ABC
xxx
End Class
然后用MemoryStream和BinaryFormatter(StreamContext要用File类型的),这样绝对是深拷贝。但是如何实现C++中的“拷贝构造”呢?
待续...