Undefined reference to `vtable' in the constructor

Question

I try to learn C++ constructors, Imma noobie yet. I wrote the next class:
screen.h

#ifndef SCREEN_H
#define SCREEN_H

#include "pch.h"

class Screen
{
public:
    Screen(const std::string& name);
    Screen(const Screen& screen);
    Screen(Screen&& screen);

    const std::string& name() const;

    virtual void draw();
private:
    std::string m_name;
};

#endif // SCREEN_H

screen.cpp

#include "screen.h"

Screen::Screen(const std::string& name)
    : m_name{name}
{
    m_name = name;
}

Screen::Screen(const Screen& screen)
    : m_name{screen.m_name}
{

}

Screen::Screen(Screen&& screen)
    : m_name{std::move(screen.m_name)}
{

}

const std::string& Screen::name() const
{
    return this->m_name;
}

But I get an issue in the compilation time:

screen.cpp:4: error: undefined reference to `vtable for Screen'

And so for all constructors.
I can't get that is my mistake... Pls, can anyone explain me?

If you have a virtual function that is not abstract, then you *must* implement it. — Some programmer dude, Apr 14 '17 at 01:42
By the way, if you have virtual functions and are meaning for the class to be polymorphic and inherited, you should really implement a (virtual) destructor. — Some programmer dude, Apr 14 '17 at 01:42
Generally a class with any virtual functions should also have a virtual destructor. — aschepler, Apr 14 '17 at 01:42
In the first constructor, `m_name = name;` is redundant given the `m_name{name}` member initializer. — aschepler, Apr 14 '17 at 01:46
@aschepler, yeah, you're right! I forgot to remove it when I tried to fix an error. Thx! — Denis Sologub, Apr 14 '17 at 01:48
@RaymondChen, yeah, it is. But I couldn't get information there due to my poor English. — Denis Sologub, Apr 14 '17 at 02:11
In which case you should say what part of it you didn't understand, instead of just repeating the question. — Raymond Chen, Apr 14 '17 at 05:27

score 5 · Answer 1 · answered Apr 14 '17 at 01:42

5

You must also implement Screen::draw.

In typical implementations, a constructor of a polymorphic class sets up a pointer to the class's "vtable", which contains pointers to the class's virtual functions. Since your first (and only) virtual function is missing, the compiler can't produce this vtable, and so all the constructors complain about it at link time.

answered Apr 14 '17 at 01:42

aschepler

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Thank you and all who commented my topic! It works perfectly! – Denis Sologub Apr 14 '17 at 01:46
@aschelper, sorry for my poor knowledges of English, but as I understood do I need to realize a `virtual` function body every time, it isn't? – Denis Sologub Apr 14 '17 at 01:50
1

A non-virtual function must be defined only if it is called. A virtual function that is not pure or is a destructor must always be defined. – aschepler Apr 14 '17 at 01:51
@aschelper, seems, I got you! Thanks for help! – Denis Sologub Apr 14 '17 at 01:53

score 4 · Accepted Answer · edited Mar 30 '22 at 12:55

What aschepler said is absolutely correct. Concerned about your begining, you may want some advice when a virtual function should be used though. Vitual function is used as a method to support polymorphism in cpp, and can be divided into two using scenario.

Interface/ abstract class

In this scenario, virtual function was declared as pure virtual, with which one class would be called as abstract class and non-instancable. By doing this, you can implement 'interface' like most modern programming support.
```
class Interface {
    //....
    virtual void f() = 0;
};
class Concrete: public Interface {
    // override this f()
    void f() override {}
};
```
Polymorphism/ concrete class

In this scenario, virtual function was declared as normal function except it can be override by derived class. And you must implement it.
```
class Parent {
    //...
    virtual void g();
}
class Derived: public Parent {
    //...
    void g() override{}
}
```

Note that you can still declare a function with the same name of parent, which was not declared as virtual function. This would be called hide, and is another topic.

Thank you for detail answer! It became for me clearer. – Denis Sologub Apr 14 '17 at 02:08 — Denis Sologub, Apr 14 '17 at 02:08

Undefined reference to `vtable' in the constructor

2 Answers2