State pattern c++ - what am i doing wrong?

Question

I just dont get what im doing wrong here. The goal is a finite state machine which changes the state and its behaviour depending on the char that it gets, but i can't get even the project structure down... here is the code i got so far, it wont complile, sorry i dont have the error codes, but one was definatley in the constructor of Automat where i tried to set the currentState to initial.

#ifndef Automat_H_
#define Automat_H_

class State;

class Automat {

public:

    Automat();
    virtual ~Automat();
    void setCurrentState(State* newCurrentState);
    void read(char c);

private:
    State* currentState;

};

#endif /* Automat_H_ */


#ifndef STATE_H_
#define STATE_H_

class Automat;

class State {
    virtual ~State();
    virtual void read(char c, Automat* m) = 0;
};

#endif /* STATE_H_ */

#ifndef INITIAL_H_
#define INITIAL_H_

#include "State.h"

class Initial: public State{
    virtual ~Initial();

    void read(char c, Automat* m);
};



#endif /* INITIAL_H_ */


//Automat.cpp

#include "../includes/Automat.h"

Automat::Automat() {
    currentState = new Initial();
}

Automat::~Automat() {
    // TODO Auto-generated destructor stub
}

void Automat::read(char c){
    //currentState->read(c, this);
}

"sorry i dont have the error codes" - so why not ask the compiler what they were? You're more likely to get help if you can narrow the code down to a [minimal test case](http://stackoverflow.com/help/mcve) and tell us exactly what goes wrong. — Mike Seymour, Apr 21 '15 at 09:34

score 1 · Answer 1 · answered Apr 21 '15 at 09:38

1

Presumably, the missing error messages told you exactly what was wrong. My compiler gives errors like

prog.cpp:29:13: error: 'virtual State::~State()' is private

because all the members of State and Initial are private - that's the default if you introduce a class definition with the class keyword.

Either add public: to these class definitions, or change the keyword to struct.

You'll also need to include "Initial.h" from "Automat.cpp", since it needs the full definition of that class.

answered Apr 21 '15 at 09:38

Mike Seymour

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Thanks for your help, I did change what you said and now im getting this: "undefined reference to vtable for initial" and the same for state. Just now i got rid of the state one by giving the deconstructor a body... that didnt do the job for the Initial... so i think it is the read() method .... do i need to implement this in the header? – mdshields Apr 21 '15 at 10:57
@mdshields: Yes, you'll have to implement `Initial::read()` (and, in general, all non-pure virtual functions). You could simply remove the `Initial` destructor, the implicitly generated one is good enough unless you need it to do anything special. The `State` constructor will also need an implementation, you can just give it an empty body in the class. – Mike Seymour Apr 21 '15 at 17:27

State pattern c++ - what am i doing wrong?

1 Answers1