Output of C++ Program | Set 8

　　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().

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　　2013-11-27　　15:41:28