c++ - I am getting LNK2005, LNK2001 and LNK1120 while using virtual function in my Program

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Learn more about Teams You have clearly realized that you need to provide an actual definition for


    Derived::Print()

, so what makes you think things are different for


    Base::Print()

? – Adrian Mole Feb 28, 2022 at 11:39 You need to add a definition of


    virtual void Print();


    Base

or make it pure virtual by writing


    virtual void Print() = 0;

instead. – user17732522 Feb 28, 2022 at 11:39 Does this answer your question? What is an undefined reference/unresolved external symbol error and how do I fix it? , specifically this answer . – user17732522 Feb 28, 2022 at 11:41 @user17732522 that did work out. I thought if you use the keyword "virtual" you had to skip the definition in the base class. – Anirudh Kanaparthy Feb 28, 2022 at 11:52 @AnirudhKanaparthy A pure virtual function doesn't need a definition, but simply a virtual function does, because you can still create a


    Base

object directly and call


    .Print()

on it. Which function definition should then be called? If you make it pure virtual, the class becomes an abstract class , for which only derived instances can be created, so the definition isn't required anymore. – user17732522 Feb 28, 2022 at 11:55

The problem is that you've only declared the virtual function Print in class Base but not defined it.

And from C++03 Standard: 10.3 Virtual functions [class.virtual]

A virtual function declared in a class shall be defined, or declared pure (10.4) in that class, or both; but no diagnostic is required (3.2).

So the way to solve this problem would be to either implement/ define the virtual member function Print in class Base or make it pure virtual by replacing the declaration with virtual void Print() = 0; inside class Base

Solution 1

Base.cpp

#include "base.h"
namespace App
    void Base::Print()
Solution 1 DEMO
Solution 2
Base.h
#include <iostream>
namespace App
    class Base
    public:
        virtual void Print() = 0;
Solution 2 DEMO
It may not have anything inside it, but it should be there.
Or indicate Print() to be defined in the derived class by making it pure:
virtual void Print() = 0;
Any one of the 2 options work. Also in your derived class it's better to make Print() as override: (Click here to know why)
class Derived : public App::Base
public:
    void Print() override;
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