Output of C++ Program | Set 8

Predict the output of following C++ programs.

Question 1

#include<iostream> using namespace std;   class Test1 {     int x; public:     void show() {  } };   class Test2 {     int x; public:     virtual void show() {  } };   int main(void) {     cout<<sizeof(Test1)<<endl;     cout<<sizeof(Test2)<<endl;     return 0; }

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

#include<iostream> using namespace std; class P { public:     virtual void show() = 0; };   class Q : public P {    int x; };   int main(void) {     Q q;     return 0; }

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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