构造函数的细节

#include<iostream>
using namespace std;
class A
{
public:
        A(){cout << "Hello world\n";}

};
class B:public A
{

};
int main()
{
        A *c = new B;//这里不能写成B *c = new A;
        return 0;
}
运行结果：
[root@bogon c++]# ./a.out
Hello world
运行结果表明执行了A的构造函数
new B的执行过程：首先调用operator new 分配足够的空间，然后执行A的构造函数，B的构造函数。
#include
using namespace std;
class A
{
public:
        A(){cout << "Hello world\n";}
        ~A() {cout << "fare\n"; }
};
class B:public A
{
public:
        B() {cout << "hi\n";}
        ~B() {cout << "Bye\n";}
};
int main()
{
        A *c = new B;
        delete c;
        return 0;
                       
}
运行结果;
[root@bogon c++]# ./a.out
Hello world
hi
fare
#include
using namespace std;
class A
{
public:
        A(){cout << "Hello world\n";}
         virtual ~A() {cout << "fare\n"; }
};
class B:public A
{
public:
        B() {cout << "hi\n";}
        ~B() {cout << "Bye\n";}
};
int main()
{
        A *c = new B;
        delete c;
        return 0;
}
运行结果：
[root@bogon c++]# ./a.out
Hello world
hi
Bye
fare
由此可见析构函数应尽量声明为虚函数！构造函数不能为虚函数。无论构造函数还是析构函数尽量不要调用虚函数！

2016.9.11
			
				
					
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					#include<iostream>
using namespace std;
class S
{
public:
    S(){cout << "Hello world";fun();}
    virtual void fun(){cout << "funS";}
    ~S(){cout << "Bye";}
};
class Derived :public S
{
public:
    Derived(){cout << "hi";}
    virtual void fun(){cout << "funDe";}
    virtual ~Derived(){cout << "farewell";}
};
int main()
{
    Derived *de = new Derived();
    delete de;
    return 0;
}
				
			
运行结果：
[root@bogon ~]# ./a.out
Hello worldfunShifarewellBye[root@bogon ~]#