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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