c++ - エラー: deque イテレータは逆参照できません

Question

算術式を中置式から後置式に変換するプログラムを作成しようとしています。「infixToPostFix」関数を呼び出さない限り、プログラムは正常に実行されます。しかし、次のコードを実行しようとすると、クラッシュして "deque iterator not dereferenceable" というエラーが発生します。逆参照演算子が見つからないため、何が問題なのかわかりません。

// infixToPostfixTest.cpp

#include "Token.h"
#include <iostream>
#include <vector>
#include <stack>
using namespace std;

// infix to postfix function prototype

void infixToPostfix(vector<Token> &infix, vector<Token> &postfix);

// priority function prototype
int priority(Token & t);

// printing a Token vector
void printTokenVector(vector<Token> & tvec);

int main() {

  // Experiment
  //--------------------------------------------------
  vector<Token> infix;

  // a + b * c - d / e % f
  //
  infix.push_back(Token(VALUE,5.0));   // a
  infix.push_back(Token(OPERATOR,'+'));
  infix.push_back(Token(VALUE,6.0));   // b
  infix.push_back(Token(OPERATOR,'*'));
  infix.push_back(Token(VALUE,7.0));   // c

  cout << "Infix expression: ";
  printTokenVector(infix);


  vector<Token> postfix;  // create empty postfix vector
  infixToPostfix(infix, postfix);  // call inToPost to fill up postfix vector from infix vector

  cout << "Postfix expression: ";
  printTokenVector(postfix);
  cout << endl << endl;

  return 0;
}

// printing a Token vector
void printTokenVector(vector<Token> & tvec)
{
    int size = tvec.size();
    for (int i = 0; i < size; i++) {
        cout << tvec[i] << " ";
    }
    cout << endl;
}




int priority(Token & t) // assumes t.ttype is OPERATOR, OPEN, CLOSE, or END
{
    char c = t.getChar();
    char tt = t.getType();

    if (c == '*'    ||    c == '/')
        return 2;
    else if (c == '+'    ||    c == '-')
        return 1;
    else if (tt == OPEN)
        return 0;
    else if (tt == END)
        return -1;
    else
        return -2;
}

void infixToPostfix(vector<Token> &infix, vector<Token> &postfix)
{
    stack<Token> stack;

    postfix.push_back(END);

    int looper = 0;
    int size = infix.size();
    while(looper < size) {
        Token token = infix[looper];

        if (token.getType() == OPEN)
        {
            stack.push(token); 
        }

        else if (token.getType() == CLOSE)
        {
            token = stack.top();
            stack.pop();

            while (token.getType() != OPEN)
            {
                postfix.push_back(token);

                token = stack.top();
                stack.pop();

            }
        }

        else if (token.getType() == OPERATOR)
        {
            Token topToken = stack.top();

            while ((!stack.empty()) && (priority(token) <= priority(topToken)))
            {
                Token tokenOut = stack.top();
                stack.pop();

                postfix.push_back(tokenOut);
                topToken = stack.top();
            }

            stack.push(token);
        }

        else if (token.getType() == VALUE)
        {
            postfix.push_back(token);
        }

        else
        {
            cout << "Error! Invalid token type.";
        }

        looper = looper + 1;
    }

    while (!stack.empty())
    {
        Token token = stack.top();
        stack.pop();

        postfix.push_back(token);
    }
}


//Token.h

#ifndef TOKEN_H
#define TOKEN_H

#include <iostream>
using namespace std;

enum TokenType { OPEN, CLOSE, OPERATOR, VARIABLE, VALUE, END };

class Token {

public:
    Token (TokenType t, char c) : ttype(t), ch(c) { }
    Token (TokenType t, double d) : ttype(t), number(d) { }
    Token (TokenType t) : ttype(t) { }
    Token () : ttype (END), ch('?'), number(-99999999) { }

    TokenType getType() {return ttype;}
    char getChar() {return ch;}
    double getNumber() {return number;}

private:
    TokenType ttype;
    char ch;
    double number;
};

ostream & operator << (ostream & os, Token & t) {

    switch (t.getType()) {
        case OPEN:
            os << "("; break;
        case CLOSE:
            os << ")"; break;
        case OPERATOR:
            os << t.getChar(); break;
        case VARIABLE:
            os << t.getChar(); break;
        case VALUE:
            os << t.getNumber(); break;
        case END:
            os << "END" ; break;
        default: os << "UNKNOWN";
    }


    return os;
}