Undefined reference to vtable

Question

I've started learning C++, I know C and Java already. I've started learning it because I want to start using object oriented programming.

However, I am stuck with code because compiler generates "undefined reference to vtable for Actor". Here you have code that generates same error, not the original one tho, because it would be less clear. I have really no idea what causes it.

struct Actor
{
     int x, y;
     virtual void move();
};

struct Player : Actor
{
     Player(int a, int b)
     {
        x = a;
        y = b;
     }

     void move();
     void draw();
};

void Player::move()
{
    ++x;
};

main()
{
    Actor *act;

    act = new Player(10, 20);
}

This question may be dumb, I don't know, I've dug everywhere but found nothing that would solve my problem.

That's a pretty crappy linker error message. What toolset generates this? — Hans Passant, Aug 18 '12 at 17:01
It's GCC. The last paragraph explains when this vague linker error shows up http://gcc.gnu.org/faq.html#vtables — Maksim Skurydzin, Aug 18 '12 at 17:09

score 9 · Accepted Answer · answered Aug 18 '12 at 16:57

9

You need to either make virtual void move(); a pure virtual function:

virtual void move() = 0;

or define Actor::move() for a base class

void Actor::move() 
{
    // do something
}

answered Aug 18 '12 at 16:57

Maksim Skurydzin

10,301
8
40
53

score 2 · Answer 2 · answered Aug 18 '12 at 16:58

2

In Actor define

virtual void move() = 0;

instead of

virtual void move();

answered Aug 18 '12 at 16:58

SiimKallas

934
11
23

score 0 · Answer 3 · answered Aug 18 '12 at 17:30

0

The error undefined reference to vtable usually means that a virtual function is declared but not defined.

answered Aug 18 '12 at 17:30

score 0 · Answer 4 · answered Sep 01 '12 at 20:45

@Maxim provided the answer in your case, however more generally speaking (from the link provided by @Maxim):

The solution is to ensure that all virtual methods that are not pure are defined. Note that a destructor must be defined even if it is declared pure-virtual.

What brought me to a grinding halt is the last sentence in the above quote, which is not obvious at first glance.

score 0 · Answer 5 · answered Feb 27 '13 at 07:47

For every class with virtual functions, an entry for class in made in vtable which further has pointers to all the virtual functions.

since there occurs a virtual void move(); declaration in base class, compiler looks for the class definition(i.e) a corresponding entry in vtable whoch is not available here as u have not defined .

so pure virtual functions will resolve the err.

Undefined reference to vtable

5 Answers5

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