(原創) 一旦為virtaul,則永世為virtual (C/C++) (.NET) (C#)
Abstract
若abstract base class將某個function設為virtual function後,則該function永世都為virtual function。
Introduction
我以前以為一個virtual function,只要繼承體系中某個class不希望其為virtual,只要不下virtual keyword即可,但這個觀念是錯誤的!!
UML
C++
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/*
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(C) OOMusou 2007 http://oomusou.cnblogs.com
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Filename : VirtualAlways.cpp
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Compiler : Visual C++ 8.0 / BCB 6.0 / gcc 3.4.2 / ISO C++
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Description : Demo virtual feature
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Release : 05/15/2007 1.0
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*/
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#include <iostream>
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using namespace std;
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class AbstractBase {
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public:
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virtual void func() = 0;
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};
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class Derived1st : public AbstractBase {
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public:
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void func() {
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cout << "Derived1st's func" << endl;
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}
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};
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class Derived2nd : public Derived1st {
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public:
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void func() {
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cout << "Derived2nd's func" << endl;
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}
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};
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int main() {
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AbstractBase& foo1 = Derived1st();
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AbstractBase& foo2 = Derived2nd();
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foo1.func();
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foo2.func();
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}

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執行結果


儘管20行



已經沒加virtual了
但27行



仍然可以繼續override,且結果也顯示override成功,這顯示了,一旦為virual function,則永世為virtual function。事實上,在UML也反映出這個原則,一個斜體的virtual function被繼承時,到了derived class仍是斜體。
C#雖然語法稍微不同,但原則仍然相同。
C#
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/*
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(C) OOMusou 2007 http://oomusou.cnblogs.com
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Filename : AbstractAlways.cs
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Compiler : Visual Studio 2005 / C# 2.0
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Description : Demo abstract feature
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Release : 05/15/2007 1.0
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*/
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using System;
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abstract class AbstractBase {
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public abstract void func();
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}
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class Derived1st : AbstractBase {
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override public void func() {
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Console.WriteLine("Derived1st's func");
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}
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}
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class Derived2nd : Derived1st {
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override public void func() {
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Console.WriteLine("Derived2nd's func");
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}
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}
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class main {
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public static void Main() {
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AbstractBase foo1 = new Derived1st();
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AbstractBase foo2 = new Derived2nd();
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foo1.func();
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foo2.func();
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}
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}

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執行結果


11行



C#的abstract,就是C++的pure virtual,且必須在class加上abstract。
16行



C#則規定一定要加上override,C++則不用。
Conclusion
C++和C#僅是語法上的小差異,但精神相同。