【c++题目003】关于析构

Which statement is false?

a) Destructors are called when a pointer is deleted.

b) Destructors are called when an object goes out of scope.

c) Destructors are called when a reference goes out of scope.

d) A virtual destructor should be used if the class has virtual methods.

e) Base class destructors are called after derived class destructors.

 

/*
关于析构
Which statement is false?
a) Destructors are called when a pointer is deleted.
b) Destructors are called when an object goes out of scope.
c) Destructors are called when a reference goes out of scope.
d) A virtual destructor should be used if the class has virtual methods.
e) Base class destructors are called after derived class destructors
*/
#include "stdafx.h"
#include <iostream>
#include <string>
#include <assert.h>
using namespace std;

class Parent
{
public:
	virtual ~Parent()
	{
		cout << "~Parent" << endl;
	}
	virtual void func(){
		cout << "Parent->func"<<endl;
	}
};
class Son:public Parent
{
public:
	virtual ~Son()
	{
		cout << "~Son" << endl;
	}
	virtual void func(){
		cout << "Son->func" << endl;
	}
};

int _tmain(int argc, _TCHAR* argv[])
{
	//Parent * pParent = new Parent;
	//{
	//	Parent p; //超出作用域也要调用析构函数
	//}
	//cout << "delete pParent..." << endl;
	//delete pParent; // delete对象指针时会调用析构函数

	////////////////////////////////////////////////////////////////////////////
	//Parent p;
	//{
	//	Parent& rP = p; //引用超出作用域时，是不会析构对象的，只是“引用而已”
	//}


	
	//当通过子类指针析构子类对象 或者 子类对象超过作用域时
	//不管析构函数是否是虚函数，都是先执行子类的析构函数，
	//再调用父类的析构函数，这跟构造时的顺序相反
	//Son s; 
	//Son * s = new Son;
	//delete s;


	//当通过父类指针析构子类对象时,如果
	//析构函数是虚函数，则delete时，根据子类对象的虚函数表，查到要调用的
	//析构韩式是子类的版本，这时调用子类的析构函数，然后析构父类，才调用父类
	//的析构函数
	//但当析构函数不是虚函数时，将直接调用父类的析构函数，而不会调用子类的析构函数，会造成资源泄漏
        //所以一般有继承关系的类，都要将析构函数定义为虚函数
	Parent * s = new Son;
	//s->func();
	delete s;
}

 

另外，对于virtual这个关键词，一般在父类中的方法定义了之后，子类不需再写virtual也是虚函数，但为了清楚起见，凡是虚函数都最好写上virtual