C++ Undefined abstract class constructor - c++

Question

I've been having a bit of trouble lately in C++ with virtual methods/inheritance
At first if i try to extend a super class i get this error:
'Undefined reference to GameState::GameState()'
but if i remove the constructors all together i don't get the above error but i end up with a segmentation fault which occurs when calling the virtual method.
This is my code:
#ifndef GAMESTATE_H
#define GAMESTATE_H
#include <stdlib.h>
#include "Resources.h"
#include "Renderer.h"
class GameState {
public:
GameState();
virtual void init(Resources *res) = 0;
virtual void exit() = 0;
virtual void update() = 0;
virtual void render(Renderer *renderer) = 0;
};
#endif // GAMESTATE_H
and this is the subclass:
#include "GameState.h"
class MainGameState : public GameState {
public:
MainGameState() : GameState() {
}
virtual void init(Resources *res) {
}
virtual void update() {
printf("test\n");
}
virtual void render(Renderer *renderer) {
}
virtual void exit() {
}
private:
SDL_Surface *image;
};

Answer 1

Try
GameState() { }
instead of
GameState();
or define it somewhere else.
Though, I'm not sure if it has anything to do with virtual methods. I think there's a bug elsewhere. Perhaps, uninitialized pointer?

Answer 2

You need to provide an implementation for the constructor if you want to explicitly call it:
class GameState {
public:
GameState() {} // implementation
virtual void init(Resources *res) = 0;
virtual void exit() = 0;
virtual void update() = 0;
virtual void render(Renderer *renderer) = 0;
};

Answer 3

Here is a complete program which implements the base and derived classes you are looking for. You receive the linker error message because you never implemented GameState::GameState. It isn't clear why you got the segmentation fault.
Remember also, if you want to delete your objects polymorphically, declare and define a virtual base-class destructor.
#include <iostream>
#define X() (std::cout << __FUNCTION__ << "\n")
class GameState {
public:
GameState() { X(); }
virtual ~GameState() { X(); }
virtual void F() = 0;
};
class MainGameState : public GameState {
public:
MainGameState() : GameState() { X(); }
void F() { X(); }
~MainGameState() { X(); }
};
int main () {
GameState* pGS = new MainGameState;
X();
pGS->F();
delete pGS;
}

Answer 4

This error 'Undefined reference to GameState::GameState()' tells you that GameState constructor isn't defined. It has nothing to do with virtual method.
Do what #Luchian and #Micheal said:
class GameState {
public:
GameState() {} // implementation
If it was a virtual method error, you would get something like "the 'MainGameState' must implement inherited pure virtual method 'method name'"
EDIT: Note that the super class constructor is implicitly called when you instantiate a sub-class.

Answer 5

The compiler will call a parameterless base class constructor (default constructor) for you anyway, so it is unnecessary. However, are you even providing a definition for GameState::GameState()? Probably not, so you should just get rid of it or provide an implementation (i.e., GameState() { }).
We need to see how you are calling the function. That looks fine to me, your bug is somewhere else. Are you perhaps declaring a pointer to an object, not initializing it, and then calling update()?
If I have to guess (and apparently I do...) I would bet that you have something like this:
MainGameState *state;
state->update(); // oops! Not initialize!
Of course I may be wrong, but like I said, I am forced to guess until you update your question with the real code.