Output of C++ Program

　　Predict the output of following C++ programs.

Question 1

 1 #include<iostream>

 2 using namespace std;

 3

 4 class Test1

 5 {

 6     int x;

 7 public:

 8     void show()

 9     {

10     }

11 };

12

13 class Test2

14 {

15     int x;

16 public:

17     virtual void show()

18     {

19     }

20 };

21

22 int main(void)

23 {

24     cout<<sizeof(Test1)<<endl;

25     cout<<sizeof(Test2)<<endl;

26     return 0;

27 }

　　Output:
　　4
　　8
　　There is only one difference between Test1 and Test2. show() is non-virtual in Test1, but virtual in Test2. When we make a function virtual, compiler adds an extra pointer vptr to objects of the class. Compiler does this to achieve run time polymorphism (See chapter 15 of Thinking in C++ book for more details). The extra pointer vptr adds to the size of objects, that is why we get 8 as size of Test2.

Question 2

 1 #include<iostream>

 2 using namespace std;

 3 class P

 4 {

 5 public:

 6     virtual void show() = 0;

 7 };

 8

 9 class Q : public P

10 {

11     int x;

12 };

13

14 int main(void)

15 {

16     Q q;

17     return 0;

18 }

　　Output: Compiler Error
　　We get the error because we can’t create objects of abstract classes. P is an abstract class as it has a pure virtual method. Class Q also becomes abstract because it is derived from P and it doesn’t implement show().

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

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　　2013-11-27　　15:41:28

巴特西

Output of C++ Program | Set 8

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