构造函数的细节

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1. #include<iostream>
   
2. using namespace std;
   
3. class A
   
4. {
   
5. public:
   
6.         A(){cout << "Hello world\n";}
   
7. 
   
8. };
   
9. class B:public A
   
10. {
    
11. 
    
12. };
    
13. int main()
    
14. {
    
15.         A *c = new B;//这里不能写成B *c = new A;
    
16.         return 0;
    
17. }
18. 运行结果：
19. [root@bogon c++]# ./a.out
    Hello world
20. 运行结果表明执行了A的构造函数
21. 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。
    
    
    
    
    
22. #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;
                           
    }
    
23. 运行结果;
24. [root@bogon c++]# ./a.out
    Hello world
    hi
    fare
25. #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;
    }
26. 运行结果：
27. [root@bogon c++]# ./a.out
    Hello world
    hi
    Bye
    fare
28. 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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    1. #include<iostream>
       
    2. using namespace std;
       
    3. class S
       
    4. {
       
    5. public:
       
    6.     S(){cout << "Hello world";fun();}
       
    7.     virtual void fun(){cout << "funS";}
       
    8.     ~S(){cout << "Bye";}
       
    9. };
       
    10. class Derived :public S
        
    11. {
        
    12. public:
        
    13.     Derived(){cout << "hi";}
        
    14.     virtual void fun(){cout << "funDe";}
        
    15.     virtual ~Derived(){cout << "farewell";}
        
    16. };
        
    17. int main()
        
    18. {
        
    19.     Derived *de = new Derived();
        
    20.     delete de;
        
    21.     return 0;
        
    22. }
    运行结果：
29. [root@bogon ~]# ./a.out
    Hello worldfunShifarewellBye[root@bogon ~]#