C++ constructor bug

Question

This is a part of my code : Declaration in the .h file :

virtual bool operator==(const File& file) const = 0;

and in the .cpp file

bool File::operator==(const File& file) const {
    return true;
}

I get this compilation error :

 1>c:\users\talw\desktop\hw5\hw5\project1\main.cpp(76): error C2259: 
              'Directory' : cannot instantiate abstract class
 1>          due to following members:  
 1>          'bool File::operator ==(const File &) const' : is abstract
 1>          c:\users\talw\desktop\hw5\hw5\project1\file.h(57) : see  
              declaration of 'File::operator =='

Please help. Thanks

So... Why did you put that `= 0` in the operator declaration? — AnT stands with Russia, Jun 20 '15 at 07:11
Before asking yet another similar question, just throw the error ("cannot instantiate abstract class") at a random search engine, which will turn up hundreds of results. — Ulrich Eckhardt, Jun 20 '15 at 07:15
[The Definitive C++ Book Guide and List](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) might be of help here — Ivan Aksamentov - Drop, Jun 20 '15 at 07:22

Vinay Shukla · Answer 1 · 2015-06-20T07:23:46.787

2

This is a pure virtual operator overloading making your class abstract so you can have the function implementation in the same class but cannot instantiate an abstract class which is giving you the below error.

cannot instantiate abstract class

A derived class of the abstract File class can have implementation of this function and can be instantiated.

edited Jun 20 '15 at 07:23

answered Jun 20 '15 at 07:13

Vinay Shukla

1,818
13
41

If you try to reproduce the exact error message (it's MSVC), you'll discover that OP has a problem with the derived class `Directory`. – Christophe Jun 20 '15 at 11:16

score 1 · Answer 2 · answered Jun 20 '15 at 11:09

1

Apparently its a copy-paste mistake at your end. Remove the =0 and you're good to go.

It is causing your Directory class to become Abstract Base Class, hence you cannot have an instance for it.

answered Jun 20 '15 at 11:09

Asim

452
5
18

1

And in this case also `Directory::operator==` instead of `File::operator==` ;-) – Christophe Jun 20 '15 at 11:20

score 0 · Answer 3 · answered Jun 20 '15 at 11:12

What does the error message mean ?

Your MSVC error message corresponds to the precise case where you derive a class Directory from an abstract class File:

1>c:\users\talw\desktop\hw5\hw5\project1\main.cpp(76): error C2259: 
              'Directory' : cannot instantiate abstract class
1>          due to following members:

The compiler explains you that you've inherited from an abstract member function that is not overriden in your new class:

1> 'bool File::operator ==(const File &) const' : is abstract 1> c:\users\talw\desktop\hw5\hw5\project1\file.h(57) : see
declaration of 'File::operator =='

How to fix it ?

To make your Directory class a concrete class, you must first declare operator== in the class to override it (note that the keyword override is optional):

class Directory : public File {
//...
public: 
    bool operator==(const File& file) const override ;
};

And then you shall provide the definition for the DERIVED class:

bool Directory::operator==(const File& file) const {
    return true;
}

Is this the only solution ?

If your intent was however to really define the virtual function for the class File, then you simply have to remove the =0 for operator==in your class definition. File::operator== will then be the default implementation of the virtual function for all the derived classes, unless they override it with a more specific one.

C++ constructor bug

3 Answers3