Predict the output of the following program?

1 #include
     
       2 using namespace std; 3   4 class Empty  5 { 6 }; 7   8 int main() 9 {10   cout << sizeof(Empty) << endl;11   return 0;12 }
     

 　　Output：　　1

　　

　　Size of an empty class is not zero. It is 1 byte generally. It is non-zero to ensure that the two different objects will have different addresses.

　　See the following example.

1 #include
     
       2 using namespace std; 3  4 class Empty  5 { 6 }; 7  8 int main() 9 {10     Empty a, b;11     12     if (&a == &b)13     {14         cout << "impossible " << endl;15     }16     else17     {18         cout << "Fine " << endl;19     }20     21     return 0;22 }
     

　　Output: 　　Fine

 

　　For the same reason (different objects should have different addresses), “new” always returns pointers to distinct objects.

　　See the following example.

 

 

1 #include
     
       2 using namespace std; 3   4 class Empty  5 { 6 }; 7   8 int main() 9 {10     Empty* p1 = new Empty;11     Empty* p2 = new Empty;12  13     if (p1 == p2)14     {15         cout << "impossible " << endl;16     }17     else18     {19         cout << "Fine " << endl;20     }21     return 0;22 }
     

　　Output：　　Fine

 

　　Now guess the output of following program (This is tricky).

1 #include
     
       2 using namespace std; 3   4 class Empty  5 { 6 }; 7   8 class Derived: Empty  9 { 10     int a; 11 };12  13 int main()14 {15     cout << sizeof(Derived);16     return 0;17 }
     

　　Output (with GCC compiler):  　　4

　　

　　Note that the output is not greater than 4. There is an interesting rule that says that an empty base class need not be represented by a separate byte. So compilers are free to make optimization in case of empty base classes.

　　As an excercise, try the following program on your compiler.

1 #include 
     
       2 using namespace std; 3    4 class Empty 5 { 6 }; 7    8 class Derived1 : public Empty 9 {10 };11   12 class Derived2 : virtual public Empty13 {14 };15   16 class Derived3 : public Empty17 {    18     char c;19 };20   21 class Derived4 : virtual public Empty22 {23     char c;24 };25   26 class Dummy27 {28     char c;29 };30   31 int main()32 {33     cout << "sizeof(Empty) " << sizeof(Empty) << endl;34     cout << "sizeof(Derived1) " << sizeof(Derived1) << endl;35     cout << "sizeof(Derived2) " << sizeof(Derived2) << endl;36     cout << "sizeof(Derived3) " << sizeof(Derived3) << endl;37     cout << "sizeof(Derived4) " << sizeof(Derived4) << endl;    38     cout << "sizeof(Dummy) " << sizeof(Dummy) << endl;39   40     return 0;41 }
     

　　运行结果如下所示。

　　

 

　　补充：

　　运行如下代码，运行结果为：　　1

1 #include    
     
       2 #include    
      
        3 using namespace std; 4 class Test 5 { 6     int arr[0]; 7 }; 8  9 int main()10 {11     Test t;12     cout<
      
     

 

 

 

 

　　Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

 

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　　2013-11-25　　20:39:10