未定义的引用错误:“类名::类成员”

为什么我的程序会出现这些错误？

undefined reference to `PizzaOrder::toppings_offered'
undefined reference to `PizzaOrder::arraySize'
undefined reference to `PizzaOrder::base_price'
undefined reference to `PizzaOrder::MedPizBase'
undefined reference to `PizzaOrder::base_price'
undefined reference to `PizzaOrder::LargePizBase'

。

这是我的程序：

#include <iostream>
#include <string>
#include <sstream>
using namespace std;
class PizzaOrder
{
private:
  int size;
  int num_toppings;
  string toppings[100];
public:
  static const string toppings_offered[];
  static const double topping_base_cost;
  static const double base_price;
  static const string defaultSize;
  static const double MedPizBase;
  static const double LargePizBase;
  static const int arraySize;
  PizzaOrder();
  PizzaOrder(int size);

 // MUTATORS & ACCESSORS & METHODS
  int GetSize() { return size; }
  int GetNumToppings() { return num_toppings; }
  string GetToppings();
  void GetPrice();
  bool SetSize(int size);
  void AddTopping(string topping);
  void AddTopping(int n); //int represents position in array toppings_offered[]
  string StringizeSize(int size);
  void DisplayPizza();

};
const string toppings_offered[] = {"1. Onions", "2. Bell Peppers",
                                   "3. Olives", "4. Pepperoni", "5. Sausage",
                                   "6. Mushrooms", "7. Jalapenos"};
const string defaultSize = "Small ";
const double topping_base_cost = 1;
const double base_price = 7;
const double MedPizBase = 0.15;
const double LargePizBase = 0.25;
const int arraySize = sizeof(toppings_offered)/sizeof(toppings_offered[0]);
PizzaOrder::PizzaOrder()
{
  SetSize(0);
}
PizzaOrder::PizzaOrder(int size)
{
  if (!SetSize(size))
     SetSize(0);
}
string PizzaOrder::StringizeSize(int size)
{
  string pizzaSize;
  if (size == 0)
     pizzaSize = "Small";
  if (size == 1)
     pizzaSize = "Medium";
  if (size == 2)
     pizzaSize = "Large";
  return pizzaSize;
}
bool PizzaOrder::SetSize(int size)
{
  if (size != 0 && size != 1
&& size != 2)
     return false;
  this->size = size;
  return true;
}
void PizzaOrder::AddTopping(string topping) // totally wrong
{
  for (int i = 0; i < arraySize; i++)
  {
     if (topping == toppings_offered[i])
     {
        toppings[num_toppings] = topping;
        num_toppings++;
     }
  }
}
void PizzaOrder::AddTopping(int n) //increments n by 1 for each valid topping chosen
{
   if (n > 0 && n < 7)
     n++;
     n = num_toppings;
}
string PizzaOrder::GetToppings()
{
  string result;
  for(int i = 0; i < GetNumToppings(); i++)
  {
     result += toppings[i];
  }
  return result;
}
void PizzaOrder::DisplayPizza()
{
   cout << "Your pizza order: " << PizzaOrder::StringizeSize(GetSize()) << ", "
        << PizzaOrder::GetNumToppings() << ", "
        << PizzaOrder::GetToppings();
}
void PizzaOrder::GetPrice()
{
   double TotalPizPrice;
   double MedPizzaPrice = base_price+(base_price*MedPizBase);
   double LargePizzaPrice = base_price+(base_price*LargePizBase);
   double topPrice = (GetNumToppings()*topping_base_cost);
   if (GetSize() == 0)
      TotalPizPrice = topPrice+base_price;
   if (GetSize() == 1)
      TotalPizPrice = topPrice+MedPizzaPrice;
   if (GetSize() == 2)
         TotalPizPrice = topPrice+LargePizzaPrice;
   cout << "Your pizza's total price is: $" << TotalPizPrice;
}
int main()
{
  PizzaOrder pizza;
  string choice;
  char selection;
  int topChoice;

  do
  {
     cout << "Size of pizza (Small, Medium, Large) or Quit?n" << endl;
     getline(cin, choice);
     selection = choice[0];
     if (selection == 'S' || selection == 's')
        pizza.SetSize(0);
     if (selection == 'M' || selection == 'm')
        pizza.SetSize(1);
     if (selection == 'L' || selection == 'l')
        pizza.SetSize(2);
     do
     {
        cout << "Current pizza: "
             << pizza.StringizeSize(pizza.GetSize())
             << pizza.GetToppings() << "n"
             << "Select an item by number (0 when done):n" << endl;
        for (int i = 0; i < 8; i++)
        {
            cout << toppings_offered[i] << "n";
        }
        cout <<"nSelection: ";
        cin >> topChoice;
        pizza.AddTopping(topChoice);
     }
     while (topChoice != 0);
  }
  while(selection != 'q' && selection != 'Q');
  pizza.DisplayPizza();
  pizza.GetPrice();

return 0;
}