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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