c++ - 出现未定义的类类型错​​误，但我确实创建了类并定义了它

Question

我正在为我的一门课做作业。只是我有一个 GumballMachine 类和一堆改变 GumballMachine 状态的状态类。

这是有问题的代码:

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  
 }
};

现在我在下面进一步定义了我的 GumballMachine 类:

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 抛出 C2259 和 C2027 错误，但在查看 MSDN 后，我觉得我做对了。也许我只是累了，但我似乎找不到错误/看不到我做错了什么。

非常感谢任何帮助。 :D

Answer 1

在定义类之前，您不能访问 GumballMachine 的任何成员，因此您必须将文件分成几个文件，每个文件包含一个类，或者定义您的 NoQuarterState: :insertQuarter 方法在 GumballMachine 类的定义之后:

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.
}