[C++] Getting undefined class type error but I did create the class and defined it

Question

Hi. Im working on an assignment for one of my classes. Simply I have a GumballMachine class and a bunch of State classes that change the state of the GumballMachine.

Here is the offending code:

class GumballMachine;

class State {
public:
 virtual void insertQuarter() const = 0;
 virtual void ejectQuarter() const = 0;
 virtual void turnCrank() const = 0;
 virtual void dispense() const = 0; 

protected:
 GumballMachine *GBM;
};

class NoQuarterState : public State {
public: 
 NoQuarterState (GumballMachine *GBM) {
  this->GBM = GBM;  
 }

 void insertQuarter() const {
  cout << "You inserted a quarter\n";  
  **this->GBM->QuarterInserted();** // <--- C2027 error on MSDN  
 }
};

Now further below I have defined my GumballMachine class as:

class GumballMachine {
public:
 GumballMachine(int numOfGB) {
  this->noQuarterState = new NoQuarterState(this);
  this->soldOutState = new SoldOutState(this);
  this->hasQuarterState = new HasQuarterState(this);
  this->soldState = new SoldState(this);
  this->winnerState = new WinnerState(this);
  this->count = numOfGB;
  if (0 < numOfGB) {
   this->state = this->noQuarterState;
  }
  else {
   this->state = this->soldOutState;
  }  
 }

        ... more code ...

 void QuarterInserted() {
  this->state = this->hasQuarterState;
 }

 ... more code ...

protected:
 int count;
 NoQuarterState *noQuarterState;
 SoldOutState *soldOutState;
 HasQuarterState *hasQuarterState;
 SoldState *soldState;
 WinnerState *winnerState;
 State *state;
};

Visual Studios was throwing a C2259 and C2027 error but after looking at MSDN I feel like I am doing it right. Maybe I am just tired, but I can't seem to find the error/see what I did wrong.

Much thanks to any help. :D

Answer 1

You cannot access any members of GumballMachine before you define the class, so you'll have to either split your file into several files, each containing one class, or define your NoQuarterState::insertQuarter method after the definition of the GumballMachine class:

class NoQuarterState : public State {
  public: 
    NoQuarterState (GumballMachine *GBM) {
       this->GBM = GBM;  
    }

    void insertQuarter() const; // Declaration only
};

class GumballMachine {
  public:
    ...
};

void NoQuarterState::insertQuarter() const {
  cout << "You inserted a quarter\n";  
  this->GBM->QuarterInserted(); // Works now bec. comp. has seen the def.
}

Answer 2

GumballMachine class is missing a semicolon after its definition.

Answer 3

class GumballMachine; declares the class. If you wish to dereference a pointer to an object of the class, you must first define the class. This is usually done in header files. In the class definition must declare all of it's member functions and variables, but you may define those functions wherever you like.

Answer 4

Chris, you need to adopt a more standard approach of splitting declarations into header files and definitions into module files.

class State needs to be in something like State.h and things like the definition of NoQuarterState::insertQuarter() needs to be in State.cpp.

When that is done you will know which .cpp files need to #include other header files to make them compile.

You are right to have class GumballMachine; before the declaration of class State since both State and NoQuarterState need to know that name.