从头文件中使用类变量时的误解

struct TodoItem
{
    std::string todo;
};

const int MAX_STACK_SIZE = 5;

class TodoStackArray
{
  public:
    TodoStackArray();  //confusion here<---
    bool isEmpty();  
    bool isFull();  
    void push(std::string todoItem);  
    void pop();  
    TodoItem* peek();  

    /* for grading purposes we need these following methods */  
    int getStackTop()    { return stackTop; }  
    TodoItem** getStack() { return stack; }   

  private:  
    int stackTop;  
    TodoItem* stack[MAX_STACK_SIZE];  
};  
struct TodoItem
{
    std::string todo;
};

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