Why do I get incomplete type when using forward declaration instead of #include?

Question

Here I have state_machine.h:

#ifndef STATE_MACHINE_H
#define STATE_MACHINE_H

// state machine classes

//#include "state_t.h"
class state_t;

class state_machine
{

public:
    state_machine();
    void change_state(state_t *newState);
    void process_state(int data);

private:
    state_t *_state;
};

#endif // STATE_MACHINE_H

And here is state_t.h:

#ifndef STATE_T_H
#define STATE_T_H

#include <QByteArray>
#include <QDebug>

//#include "state_machine.h"
class state_machine;

class state_t
{
public:
    state_t(QByteArray stateName);
    virtual ~state_t();
    virtual void processState(state_machine *sm, int input) = 0;

    void defaultUnknownEventHandler(int event);

    QByteArray name;
};

#endif // STATE_T_H

Then some state classes, which are all the same more or less, I'll just list one:

teststate1.h:

#ifndef TESTSTATE1_H
#define TESTSTATE1_H

#include "state_t.h"

class testState1 : public state_t
{
public:
    testState1();
    void processState(state_machine *sm, int event);
};

#endif // TESTSTATE1_H

teststate.cpp:

#include "teststate1.h"
#include "teststate2.h"

testState1::testState1() :
    state_t("state1")
{
}

void testState1::processState(state_machine *sm, int event)
{
    qDebug() << name << ": event" << event;
    switch (event)
    {
        case 2:
        {
            // error: invalid use of incomplete type 'class state_machine'
            sm->change_state(new testState2()); 
            break;
        }
        default:
        {
            defaultUnknownEventHandler(event);
            break;
        }
    }
}

The problem:

I am trying to tidy up my code and use the minimal amount of header inclusion (especially in header files) by using forward declarations. You can see in the state_machine class header I have commented out #include "state_t.h" and replaced it with a forward declaration class state_t;. This worked and my code compiled and ran.

Then, in state.h I replaced #include "state_machine.h" with a forward declaration class state_machine; (you can see where I commented it out).

But now I get the error error: invalid use of incomplete type 'class state_machine' which I have commented in testState1.cpp code. But I am not sure why. Why is state_machine an incomplete type?

Answer 1

teststate.cpp needs the definition of state_machine; so include state_machine.h there.

"Incomplete" means that the compiler has only seen a declaration, class state_machine;, not a full definition. You can do various things with an incomplete type, such as declaring pointers or references. But you can't call member functions (as you do here), or more generally do anything that requires knowledge of the class members, without the full definition.