Virtual class and inheritance in c++

Question

I am developing a new project using c++:

File A.cpp

#ifndef A_hpp
#define A_hpp

class A{

public:
    A(int at, int bt);
    ~A();
    virtual double b()=0;
    virtual double c()=0;
    virtual double g()=0;
private:
    int at;
    int bt;
};

#endif /* A_hpp */

File A.cpp

#include "A.hpp"

A::A(int at, int bt){
    this->at=at;
    this->bt=bt;
}

A::~A(){
    //no code
}

File B.hpp

#ifndef B_hpp
#define B_hpp

#include "A.hpp"
class B:public A{

public:
    B(int at, int bt, double val);
    ~B();

private:
    double *elems;
};
#endif /* B_hpp */

File B.cpp

#include "B.hpp"
#include "A.hpp"

B::B(int at, int bt, double val):A(at, bt){

    elems=new double[at*bt];

    for(int i=0;i<at*bt;i++)
        *(elems+i)=val;
}
B::~B(){
    delete []elems;
}

double A::c(){
        return *elem;
    }

double A::b(){
        return (*elem)-*(elem+1);
    }


double A::g(){
        return (*elem)*(*elem)*(*elem);
    }

I get an error, Use of undeclared identifier 'elem', when I implement these virtual function b(), c() et g() in the class B

Please copy-paste the error message, instead of trying to paraphrase it. EDIT: the lack of indentation and spacing made your code **very** hard to read. You are implementing `a`, `b`, `c` functions for your `A` class (`double A::b()`), and `A` class knows nothing about `elems`. Did you want to implement them for `B` class (`double B::b()`)? — Algirdas Preidžius, Feb 10 '17 at 18:48
Why you define A's member functions `b, c, g` in source of `B.cpp`? — Raindrop7, Feb 10 '17 at 18:59
More to the point, why do you bother defining `A`'s member functions `b, c, g` at all? You declared them as abstract. Also, you _should know_ that `A`'s member functions will not have access to members declared in `B`.... It does not matter what file you declare them in! — Disillusioned, Feb 10 '17 at 19:03
If you write `double A::c(){ ...`, you do not implement `c` in class `B` but rather `c` in class `A`; and class `A` is not aware of `elem`, which is a member of `B`, right? — Stephan Lechner, Feb 10 '17 at 19:08
@BetterEnglish All of these unnecessary pointers and `new`/`delete` mess. Use idiomatic c++ constructs please! — πάντα ῥεῖ, Feb 10 '17 at 19:09
Replace the `new[]` stuff with `std::vector` and add a virtual destructor in the base class. These two things must be repaired in the code before proceeding with any further problems. Also consider making the public functions non-virtual and make them delegate to private virtual functions instead. — Christian Hackl, Feb 10 '17 at 19:15
@BetterEnglish The way you've changed your code between edits suggests you're not copy-pasting from your local source code. Don't do that! You just create confusion when you inevitably make a hash job of it - _especially as you're very obviously not familiar with the nuances of the language. You make your own task more difficult and waste everyone's time when you don't do things properly. — Disillusioned, Feb 10 '17 at 19:20
@CraigYoung, Sorry for the confusion but i have used the same code with some correction of typo. That 's it! — LearnToGrow, Feb 10 '17 at 19:22
@πάνταῥεῖ, What do you mean by idiomatic c++ constructs! — LearnToGrow, Feb 10 '17 at 19:23
@BetterEnglish You miss the point. You wouldn't have to correct typo's if you simply copy-paste. — Disillusioned, Feb 10 '17 at 19:24
@BetterEnglish _"What do you mean by idiomatic c++ constructs!"_ Standard [container classes](http://en.cppreference.com/w/cpp/container), [smart ponters](http://en.cppreference.com/w/cpp/memory). — πάντα ῥεῖ, Feb 10 '17 at 19:33

Rama · Answer 1 · 2017-02-10T19:06:18.237

There is a typo. Change elem to elems.

Plus,

To imlement virtual inheritace you have to add the methods in the class B header:

virtual double b();
virtual double c();
virtual double g();

And in the implementation change namespace A by B in the .cpp:

double B::c(){
        return *elems;
    }
double B::b(){
        return (*elems)-*(elems+1);
    }
double B::g(){
        return (*elems)*(*elems)*(*elems);
    }

Alex Zywicki · Accepted Answer · 2017-02-10T19:13:34.463

0

The issue is that you are implementing the overrides for the base classes virtual functions incorrectly.

What your code is actually doing is implementing the pure virtual functions from A while trying to use the class member variables from B

Given your base class:

class A
{
public:
    A(int at, int bt);
    ~A();
    virtual double b()=0;
    virtual double c()=0;
    virtual double g()=0;
private:
    int at;
    int bt;
};

Your child class should be:

class B:public A
{
public:
    B(int at, int bt, double val);
    ~B();
    virtual double b();
    virtual double c();
    virtual double g();
private:
    double *elem;
};

With the implementation of B like so:

B::B(int at, int bt, double val):A(at, bt)
{
    elem=new double[at*bt];
    for(int i=0;i<at*bt;i++)
    {
        *(elem+i)=val;
    }
}
double B::a()
{
    return *elem;
}
B::~B()
{
    delete []elem;
}
double B::b()
{
    return (*elem)-*(elem+1);
}

double B::g()
{
    return (*elem)*(*elem)*(*elem);
}

edited Feb 10 '17 at 19:13

answered Feb 10 '17 at 18:54

Alex Zywicki

2,263
1
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This does not address the original problem being the assumed typo of `elem` instead of `elems`. – Gambit Feb 10 '17 at 18:55
Also making `b, c, g` pure virtual functions will make class B ADT thus you cannot instantiate it. – Raindrop7 Feb 10 '17 at 18:57
@Raindrop7 My bad, that was a Copy paste Slip up. will fix – Alex Zywicki Feb 10 '17 at 18:59
@Gambit Never mind, on second thoughts I actually have no idea what OP intends. Question seems all over the place. – Disillusioned Feb 10 '17 at 19:13
@CraigYoung A misunderstanding from all parties it seems. – Gambit Feb 10 '17 at 19:15
I think it comes down to a combination of typos in the question and a lack of understanding as far as inheritance, polymorphism and virtual functions goes. – Alex Zywicki Feb 10 '17 at 19:17
@AlexZywicki, I agree with you, I lack some fundamental c++ inheritance and virtual function. I am more comfortable with java. – LearnToGrow Feb 10 '17 at 19:20
See this list of books on C++: http://stackoverflow.com/q/388242/1865694 – Alex Zywicki Feb 10 '17 at 19:21
@BetterEnglish Regardless, that's still not an excuse to not copy-paste a simple example of your problem code to ask your question. – Disillusioned Feb 10 '17 at 19:21
@AlexZywicki, thanks so much for your constructive advice! – LearnToGrow Feb 10 '17 at 19:25

score 0 · Answer 3 · answered Feb 10 '17 at 19:11

I think you misinterpret placing an implementation in a particular B.cpp-file as "implementing something in class B". If you write double A::c(){ ..., you do not implement c in class B but rather c in class A, regardless in which file you write this implementation. And class A is not aware of elems, which is a member of B, what explains the error message Use of undeclared identifier 'elem'.

Write double B::c(){ ... instead, and also declare double c() in the definition of class B. (The same of course for the other functions)

score 0 · Answer 4 · answered Feb 10 '17 at 19:17

0

elems is a member of class B which derived from class A so you cannot access the members of a child from the parent:

double A::b(){ 
    return (*elems)-*(elems+1); // elems is a member of B not A
}

answered Feb 10 '17 at 19:17

Raindrop7

3,889
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Virtual class and inheritance in c++

4 Answers4