C# 構(gòu)造函數(shù)中調(diào)用虛函數(shù)

using System; using System.Diagnostics; using System.Text; using System.Collections; using System.Collections.Generic;class Test {public class Base{public Base(){System.Console.WriteLine("Base.Base");ABitDangerousCall();}public virtual void ABitDangerousCall(){System.Console.WriteLine("Base.ABitDangerousCall");}private class Inner{public Inner(){System.Console.WriteLine("Base.Inner.Inner");}}private Inner inner = new Inner();}class Derived : Base{public Derived(){System.Console.WriteLine("Derived.Derived");ctorInitializedMember = 5;}// ctorInitializedMember is default initialized to zero before the constructor initializes it.private int ctorInitializedMember;private int derivedInt;public override void ABitDangerousCall(){System.Console.WriteLine(String.Format("Derived.ABitDangerousCallctorInitializedMember={0} derivedInt={1}", ctorInitializedMember, derivedInt));}private class Inner{public Inner(){System.Console.WriteLine("Derived.Inner.Inner");}}private Inner inner = new Inner();}class Program{static void Main(string[] args){Derived d = new Derived();Console.Read();}}}

輸出結(jié)果：

Derived.Inner.Inner
Base.Inner.Inner
Base.Base
Derived.ABitDangerousCallctorInitializedMember=0 derivedInt=0
Derived.Derived

由于實(shí)例構(gòu)造函數(shù)的調(diào)用順序是先基類(lèi)構(gòu)造函數(shù)再子類(lèi)構(gòu)造函數(shù)，如上面例子，在基類(lèi)中調(diào)用被子類(lèi)重寫(xiě)了的虛函數(shù)ABitDangerousCall時(shí)，實(shí)際會(huì)調(diào)用子類(lèi)的ABitDangerousCall，

該函數(shù)中使用了變量ctorInitializedMember，但此時(shí)子類(lèi)的構(gòu)造函數(shù)還沒(méi)調(diào)用，故ctorInitializedMember未被賦值，這樣就出現(xiàn)了不想看到的結(jié)果。我們本來(lái)是想想看到ctorInitializedMember=5的結(jié)果，結(jié)果卻是0。

因此在基類(lèi)的構(gòu)造函數(shù)中調(diào)用虛函數(shù)是很危險(xiǎn)的，項(xiàng)目大了代碼復(fù)雜了就很容易造成極隱蔽的BUG。同時(shí)在子類(lèi)的構(gòu)造函數(shù)中調(diào)用虛函數(shù)也是要小心的，盡量避免，原因同上。

如果非要調(diào)用，那就要細(xì)心察看虛函數(shù)中的變量是否已經(jīng)全部在前面處理妥當(dāng)了

posted on 2016-11-11 18:39 時(shí)空觀察者9號(hào) 閱讀(...) 評(píng)論(...) 編輯 收藏

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