Resolving circular dependency in the State Pattern with C++

Question

I'm trying to implement a State Pattern in C++, but have problems with the circular dependency. I have read other related material here - unfortunately it didn't help me. I don't have a lot of experience with C++, so bear with me. The following code is developed on a Ubuntu 10.10 machine in Eclipse Helios CDT:

ConcreteSystem.h

#ifndef CONCRETESYSTEM_H_
#define CONCRETESYSTEM_H_

class SystemState;

class ConcreteSystem {
public:
    ConcreteSystem();
    void SelfTestFailed();
    void Restart();
private:
    friend class SystemState;
    SystemState *currentState;
    void ChangeState(SystemState *state);
};

#endif /* CONCRETESYSTEM_H_ */

ConcreteSystem.cpp

#include "ConcreteSystem.h"
#include "SystemState.h"

ConcreteSystem::ConcreteSystem() {
    currentState = SelfTest::GetInstance();
}

void ConcreteSystem::SelfTestFailed() {
    currentState->SelfTestFailed(this);
}

void ConcreteSystem::Restart() {
    currentState->Restart(this);
}

void ConcreteSystem::ChangeState(SystemState *state){
    currentState = state;
}

SystemState.h

#ifndef SYSTEMSTATE_H_
#define SYSTEMSTATE_H_

class ConcreteSystem;

class SystemState {
public:
    virtual void Restart(ConcreteSystem *cs);
    virtual void SelfTestFailed(ConcreteSystem *cs);
protected:
    virtual void ChangeState(ConcreteSystem *cs, SystemState *state);
};

#endif /* SYSTEMSTATE_H_ */

SystemState.cpp

#include "SystemState.h"
#include "ConcreteSystem.h"

void SystemState::Restart(ConcreteSystem *cs) {

}

void SystemState::SelfTestFailed(ConcreteSystem *cs) {

}


void SystemState::ChangeState(ConcreteSystem *cs, SystemState *state) {
    cs->ChangeState(state);
}

SelfTest.h

#ifndef SELFTEST_H_
#define SELFTEST_H_

#include "SystemState.h"


class SelfTest : public SystemState {
public:
    SelfTest();
    void SelfTestFailed(ConcreteSystem* cs);
    static SystemState* GetInstance();
private:
    static SystemState* instance;
};

#endif /* SELFTEST_H_ */

SelfTest.cpp

#include "SelfTest.h"
#include "Failure.h"

SystemState* SelfTest::instance = 0;

SelfTest::SelfTest() {

}

void SelfTest::SelfTestFailed(ConcreteSystem *cs) {
    ChangeState(cs, Failure::GetInstance());
}

SystemState* SelfTest::GetInstance() {
    if (instance == 0) {
        instance = new SelfTest();
    }
    return instance;
}

Failure.h

#ifndef FAILURE_H_
#define FAILURE_H_

#include "SystemState.h"

class SelfTest;

class Failure : public SystemState {
public:
    Failure();
    void Restart(ConcreteSystem* t);
    static SystemState* GetInstance();
private:
    static SystemState* instance;
};

#endif /* FAILURE_H_ */

Failure.cpp

#include "Failure.h"
#include "SelfTest.h"

SystemState* Failure::instance = 0;

Failure::Failure() {

}

void Failure::Restart(ConcreteSystem* t) {
    ChangeState(t, SelfTest::GetInstance());
}

SystemState* Failure::GetInstance() {
    if (instance == 0) {
        instance = new Failure();
    }
    return instance;
}

I have problem with the includes, which gives me some weird compiler errors. Anyone with a good solution to this problem?

Answer 1

From the looks of the code you've posted, you'll have classes being redefined. Looking at your Failure.cpp file, you have:

#include "Failure.h"
#include "SelfTest.h"

Which will include both of those files, and each of those files include the SystemState.h file. Since the SystemState.h file is included more than once, it tries to redefine the SystemState class. At the top of each of your header files, you should do something like this:

// SystemState.h
#ifndef SystemState_h
#define SystemState_h
.. class definition ..
#endif // close the if statement from above.

As an aside on the design, I think it's bad form for the states to know about each other - use your ConcreteSystem as a state controller and then base the state on the return value of the last state operation.

Also, if you're relatively inexperienced with C++, I would recommend looking at this as a great source of learning material (in addition to StackOverflow, of course!).