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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