Understanding Inheritance in C++ [duplicate]

Question

I am trying to learn C++, and wrote this code. According to my understaing this code need to produce output as "Derived Class" but output is "Base Class". Please help me understand this.

#include <iostream> using namespace std; class Base { public: char* name; void display() { cout << name << endl; }

};

class Derived: public Base { public: char* name; void display() { cout << name << ", " << Base::name << endl; } }; int main() { Derived d; d.name = "Derived Class"; d.Base::name = "Base Class"; Derived* dptr = &d; Base* bptr = dptr; bptr->display(); }

Please consider me as a beginner and explain why its output is "Base Class"

AndyG · Accepted Answer · 2014-04-04 06:45:35Z

2

You need to make the display() method virtual

Like this:

class Base{ public: char* name; virtual void display() { cout << name << endl; }

virtual allows derived classes to 'override' the base class' functions.

answered Apr 4, 2014 at 6:45

AndyG

41.4k8 gold badges122 silver badges155 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

someone Over a year ago

@andy.. After making it virtual its output is "Derived Class Base Class"...but I am expecting only "Derived Class"

AndyG Over a year ago

@someone: And why shouldn't it? You wrote cout << name << ", " << Base::name << endl; not cout << name << endl

user93353 · Accepted Answer · 2014-04-04 06:46:42Z

http://www.parashift.com/c++-faq/dyn-binding.html

Non-virtual member functions are resolved statically. That is, the member function is selected statically (at compile-time) based on the type of the pointer (or reference) to the object.

In contrast, virtual member functions are resolved dynamically (at run-time). That is, the member function is selected dynamically (at run-time) based on the type of the object, not the type of the pointer/reference to that object.

R Sahu · Accepted Answer · 2014-04-04 06:47:58Z

If you want a derived class function to be called through a base class pointer, you need to make the base class function a virtual function. Add the key word virtual to the function and your code will be good to go.

Community · Accepted Answer · 2017-05-23 12:07:28Z

C++ dispatch is little bit weird. Unless you declare the display method 'virtual' the bptr->display would alway call the display of base class.

More detailed explanation is here

stefaanv · Accepted Answer · 2014-04-04 07:00:53Z

Polymorphism (working generically with base class pointers pointing to instances of derived classes) is done by dispatching to virtual functions as explained in the other answers. However, if you point to a derived class with a base class pointer, you will use the data members of the base class because the compiler doesn't know the data members of the derived class and same goes for calling functions of the base class, so the base class display() function uses the base class name data member and you result is "Base Class".

Collectives™ on Stack Overflow

Understanding Inheritance in C++ [duplicate]

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Linked

Related