关于构造函数中的多态性的一个小问题 [英] a little question about polymorphism in constructor

问题描述

class A{
public:
	A(){msg();}
	virtual void msg(){AfxMessageBox(_T("A"));}
};

class B: public A{
public:
	B():A(){}
private:
	virtual void msg(){AfxMessageBox(_T("B"));}
};

void test(){
	B b;	//will Message("A");  -------why? I think I can get Message("B")!-----

	A*pA=&b;
	pA->msg();//will Message("B");
}

推荐答案

Quote：

B b; //将消息（A）; - - - -为什么？我想我可以得到消息（B）！-----

B b; //will Message("A"); -------why? I think I can get Message("B")!-----

B 构造函数明确地调用一个构造函数，因此你得到这样一条消息。

对不起，这是错误的，因为 pasztorpisti 指出。您可以在这里找到正确的信息：

C ++ FAQ：[23.5]当我的基类的构造函数在其对象上调用虚函数时，为什么不调用我的派生类对该虚函数的重写？ [ ^ ]

B constructor explicitely calls the A constructor, hence you get such a message.
Sorry, that was wrong, as pasztorpisti pointed out. You may find correct info here:
C++ FAQ: [23.5] When my base class's constructor calls a virtual function on its this object, why doesn't my derived class's override of that virtual function get invoked?[^]

Quote：

pA- > msg（）; //将消息（B）;

pA->msg();//will Message("B");

这是通常的polimorphic行为。

This is the usual polimorphic behaviour.

编辑：Richard MacCutchan提出了写作的好主意关于这个问题的提示/技巧。我刚刚用一个关于这个问题的示例代码进行了一个小讨论：永远不要在C ++ / C＃/ Java中从类的构造函数或析构函数中调用虚方法！ [ ^ ]



我懒得写这里是答案，因为我已经回答了几次这个问题。原因如下：永远不要直接或间接地从（基类）构造函数中调用虚方法。请阅读我的帖子：



http://www.codeproject.com/Messages/4619258/Re-Serial-Port-OK-in-class-constructor-but-not-out.aspx [ ^ ]

http://www.codeproject.com/Messages/4619708/Re-Serial-Port-OK-in-class-constructor-but-not-out.aspx [ ^ ]

Richard MacCutchan came up with a good idea of writing a tip/trick about this problem. I've just put together a small discussion with example code about this problem: Never call virtual methods from the constructor or destructor of your classes in C++/C#/Java![^]

I am lazy to write here the answer again as I have already answered this question a few times. The reason in a few words: Never call virtual methods directly or indirectly from (base class) constructors. Please read my posts regarding this:

http://www.codeproject.com/Messages/4619258/Re-Serial-Port-OK-in-class-constructor-but-not-out.aspx[^]
http://www.codeproject.com/Messages/4619708/Re-Serial-Port-OK-in-class-constructor-but-not-out.aspx[^]

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