stack - C++: Deque Iterator Not Dereferenceable; can't find the rogue pop/top -

i'm doing similar user did: error: deque iterator not dereferenceable

i've been looking age cannot see i'm going wrong. solution other poster finding place tried pop or top deque 0 elements. can't find i'm doing in code.

edit: suspect issue within syalg or osprocess, if helps.

// testcalculator.cpp : main project file.  #include <string> #include <iostream> #include <locale> #include <ctype.h> #include <vector> #include <deque>  using namespace system; using namespace std; //using std::string;  bool lastchardigit = true; string rawstring; //contains raw equation user types in.  deque<string> tokenequation(1); //contains equation in tokenised infix form. deque<string> rpnequation; //contains equation in tokenised rpn form. deque<string> operatorstack; //used part of shunting yard algorithm deque<string> solverstack; //used solve rpn equation.  locale loc; //used verify digits. //start function declaration int main(); void tokeniser(string rawequation); void syalg(); void osprocess(string newoperator); void solver(); //end function declaration   int main() {     cout << "please enter valid infix notation equation, without parenthesis.\n";     cin >> rawstring;     tokeniser(rawstring);     cout << "\n";     system("pause");     return 0; }  void tokeniser(string rawequation) {     int testcharpos = -1; // initialise index of raw string     int tokenvectorpos = 0; // initialise token array position     int tokenvectorprintpos = 0; // initialise print position      (int elength = rawequation.length(); elength != 0; elength--) // each character in raw string...     {         testcharpos++; // increment char we're testing         char testchar = rawequation.at(testcharpos); // establish current test char          if (isdigit(testchar, loc)) //if testchar digit         {             if (lastchardigit) //if last character digit             {                 tokenequation[tokenvectorpos] += testchar; //append tested char current token array pos             }             if (!lastchardigit) //if last character not digit             {                 tokenequation.push_back(string(1, testchar)); //establish new element testchar in it.                 tokenvectorpos++;             }             lastchardigit = true;         }          if (!isdigit(testchar, loc))//if testchar not digit         {             tokenequation.push_back(string(1, testchar)); //establish new element testchar in it.             tokenvectorpos++;             lastchardigit = false;         }     }      cout << "the tokens of equation are:\n\n"; //outputs tokens testing purposes.      (int tokenlength = tokenequation.size(); tokenlength != 0; tokenlength--)     {         cout << "     " << tokenequation[tokenvectorprintpos];         cout << "\n";         tokenvectorprintpos++;     }      syalg(); //call syalg. }  void syalg() //this function uses shunting yard algorithm convert infix tokens rpn. {     cout << tokenequation.size();     (int testtokenlength = tokenequation.size(); testtokenlength != 0; testtokenlength--) //for each token in tokenised deque     {         if (isdigit(tokenequation.front().at(0), loc)) //check if it's number         {             rpnequation.push_back(tokenequation.front()); //add first raw token rpn equation             tokenequation.pop_front(); //pop token deque         }         if (!isdigit(tokenequation.front().at(0), loc)) //if it's operator         {             osprocess(tokenequation.front()); //run syalg operator stack procedure. nb pop front of tokenequation you.         }     }       cout << "the tokens of equation are:\n\n"; //outputs tokens testing purposes.     int rpnprintpos = 0;     (int tokenlength = rpnequation.size(); tokenlength != 0; tokenlength--)     {         cout << "     " << rpnequation[rpnprintpos];         cout << "\n";         rpnprintpos++;     } }  void osprocess(string newoperator) //this function processes operator stack {     bool pushednewoperator = false;     std::string newopstd = newoperator; //creates std::string version of argument easier comparison.     while (pushednewoperator == false){ //as long new operator still waiting go stack         if (!operatorstack.empty()) //if there's operator on stack         {             if (newopstd == "/" || "*")             {                 std::string osbackstd = operatorstack.back(); //create std version of of opstack comparison.                 if (osbackstd == "+" || "-")                 {                     operatorstack.push_back(newoperator); //add tested operator stack                     tokenequation.pop_front(); //and pop token equation                     pushednewoperator = true; //set flag variable true stop looping                 }                 else                 {                     rpnequation.push_back(operatorstack.back()); //add top of operator stack equation                     operatorstack.pop_back(); //pop                 }             }             else             {                 rpnequation.push_back(operatorstack.back()); //add top of operator stack equation                 operatorstack.pop_back(); //pop             }         }         if (operatorstack.empty())         {             operatorstack.push_back(newoperator); //add tested operator stack             tokenequation.pop_front(); //and pop token equation             pushednewoperator = true; //set flag variable true stop looping         }     }     //for each operator on stack, until following statement returns false...     //check if precedence of newoperator less or equal top operator. }  void solver() //this function solves rpnequation {     //push each token solver stack     //if push operator, solve against stack     //when rpn equation empty , solver stack has 1 token in it, have solution } 

one major issue multitude of lines if (newopstd == "/" || "*"), or effect. these need changed if (newopstd.compare("/") == 0 || newopstd.compare("*") == 0).

i think these checks failing means while loop they're in turns while(true).