Why must I re-declare a virtual function from an inherited class?

Question

I\'m working on a simple C++ program and am having a difficult time understanding a compiler error I was getting. The issue was caused by me attempting to create a derived class

Answer 1

A very non-intuitive reason why overridden virtual methods must be derived in the base class stems from the fact that C++ allows different parts of the class to be placed into different files, into different translation units.

With some other languages (I'm looking in Java's direction), a single class must be placed in a single file. This is not true with C++. It is perfectly legal for a class to have some of its methods declared in one translation unit, and other methods declared in another translation unit, which could be in a file in some different directory altogether.

Each such file gets compiled separately and individually. When compiling one translation, the C++ compiler has no knowledge of any other translation unit, any other file, that might contain other pieces of the same class.

Now let's say that you are allowed to omit an overriden virtual method from the class declaration. This creates an immediate problem: when compiling the class's constructor, it is necessary for the compiler to know whether the class overrides any virtual methods from any of the superclasses, in order to correctly assemble the virtual table dispatch for the class being constructed. Without an explicit declaration, the compiler has no way of knowing whether or not some other translation unit might define an overridden virtual method.

And that's why overridden virtual methods must be explicitly included in the class's declaration. In conclusion: because C++ formally specifies phase 9, the linkage phase, with the actual compilation carried on in earlier phases, overridden methods must be explicitly declared.

Answer 2

you should override all the base class pure virtual functions to be able to instantiate derived class.

• you cannot define a base class member function from derived class.

in your example you are trying to define method1 and method2 and method3 which are not members of DerivedClass!! you have to declare them yourself in your derived class. compiler doesn't do it for you.

so your Derivedclass.h will look like:

#ifndef DERIVEDCLASS_H
#define DERIVEDCLASS_H

#include "BaseClass.h"

class DerivedClass: public BaseClass
{

    public:
        DerivedClass(void); 

        virtual int method1(void); // not pure function
        virtual int method2(void);
        virtual float method3(void);
};

#endif // DERIVEDCLASS_H

Answer 3

It doesn't work this way. You need to declare the methods you're going to define, whether they're overriding a virtual method or not.

This isn't just an unreasonable requirement of the language. Without this you would be unable to define partially virtual class, i.e., you could have BaseSubtype that has common implementation of method1() but requires classes derived from it to implement method2() and method3()

Answer 4

When you declare a method in a derived class, you say to the compiler:

I want to override this method in this class

So if you don't declare a method in the derived class, you say:

I don't want to override this method; derived class's implementation is the same as the one in the base class

In your case, the base class declares them as pure virtual, so in this case it can be paraphrased:

I don't want to implement this method in this class

If you try to define a method but not declare it, you contradict yourself. The compiler detects that (to protect you from your own negligence).