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