cin

consider the following:

double number_value;
cin >> number_value;

If the user input 666+12

I know number_value will be only 666. (right?)
But is this a good pratice? I mean when it encounters the + sign, will there be a problem?

cause usually when I try to input a char into an int it kinda crash..

but with this code it seems to be fine?
code was made by stroustrup btw..

Yeah, it will only read 666. The reason you get an error when you enter a char[acter] is because it's defined as an int[eger]. I don't see how this + sign could hurt your program, personally I would rather user if statements/loops/or switch statements to find out when a user wants to quit or if they entered a certain value.

I'm not really sure what your trying to do, could you explain fully... (I'm sure my info didn't really help you that much, lol)

Here is the full code..
This is a clculator example from the c++ programming language by Bjarne Stroustrup
what I posted is part of the parser

Code:

/****************************************HEADERS***************************************************/

#include <iostream>
#include <sstream>
#include <string>
#include <map>
#include <cctype>
using namespace std;



/****************************************GLOBAL VARIABLES****************************************/

enum Token_value
{
      NAME,		NUMBER,		END,
      PLUS='+',	MINUS='-',		MUL='*',		DIV='/',
      PRINT=';',	ASSIGN='=',		LP='(',		RP=')',
};

struct LineCounter
{
	  LineCounter(): line(0) {}
	  unsigned long current() { return line; }
	  void new_line() { ++line; }
      private:
	  unsigned long line;
};

LineCounter line_count;

Token_value curr_tok = PRINT;
double number_value;
string string_value;
map<string,double> table;
map<string,string> expression;
int no_of_errors;
istream* input;			//pointer to input stream



/****************************************FUNCTIONS CALLS*****************************************/

double expr(bool get);
double term(bool get);
double prim(bool get);
double create();
double error(const string& msg);
Token_value get_token();


/*****************************************MAIN******************************************************/

int main(int argc, char* argv[])
{
      switch (argc)
      {
	  case 1 :				// read from standard input
	        input = &cin;
	        break;
	  case 2 :
	        input = new istringstream(argv[1]);		// read argument string
	        break;
	  default :
	        error ("Too many arguments");
	        return 1;
      }
      
      table["pi"] = 3.1415926535897932385;		// insert predefined names
      table["e"] = 2.7182818284590452354;
      
      while (*input)
      {
	  cout << "Enter a mathematical expression : (Type 'E' to exit)" << endl;
	  get_token();
	  if (curr_tok == END) break;
	  if (curr_tok == PRINT) continue;
	  cout << expr(false) <<endl;
      }
      
      if (input != &cin) delete input;
      return no_of_errors;
}



/*****************************************OTHER FUNCTIONS*****************************************/

double expr(bool get)				// add and substract
{
      double left = term(get);
      
      for(;;)
	  switch (curr_tok)
	  {
	        case PLUS :
		    left += term(true);
		    break;
	        case MINUS :
		    left -= term(true);
		    break;
	        default :
		    return left;
	  }
}



double term(bool get)				// multiply and divide
{
      double left = prim(get);
      
      for (;;)
	  switch (curr_tok)
	  {
	        case MUL :
		    left *= prim(true);
		    break;
	        case DIV :
		    if (double d = prim(true))
		    {
			left /= d;
			break;
		    }
		    return error("Divide by 0");
	        default :
		    return left;
	  }
}



double prim(bool get)				// handle primaries
{
      if (get) get_token();
      
      switch (curr_tok)
      {
	  case NUMBER :			// floating point constant
	  {
	        double v = number_value;
	        get_token();
	        return v;
	  }
	  case NAME :
	  {
	        string str = "new";
	        if (string_value == str) return create();
	        else
	        {
		    double &v = table[string_value];
		    if (get_token() == ASSIGN) v = expr(true);
		    return v;
	        }
	  }
	  case MINUS :			// unary minus
	        return -prim(true);
	  case LP :
	  {
	        double e = expr(true);
	        if (curr_tok != RP) return error("')' expected");
	        get_token();			//eat ')'
	        return e;
	  }
	  default :
	        return error("primary expected");
      }
}



Token_value get_token()
{
      char ch = 0;
      do {			// skip whitespace except '\n'
	  if(!cin.get(ch)) return curr_tok = END;
      }while (ch != '\n' && isspace(ch));
      switch (ch)
      {
	  case 0 : case  'E':
	        return curr_tok=END;
	  case ';' :	return curr_tok = PRINT;
	  case '\n' :	line_count.new_line();
	  case '*' :
	  case '/' :
	  case '+' :
	  case '-' :
	  case '(' :
	  case ')' :
	  case '=' : return curr_tok = Token_value(ch);
	  
	  case '0' :	case '1' :	case '2' :	case '3' : 	case '4' :
	  case '5' :	case '6' :	case '7' :	case '8' :	case '9' :
	  case '.' :
	        cin.putback(ch);
	        cin >> number_value;
	        return curr_tok = NUMBER;
	  default:				// NAME, NAME =, or error
	        if (isalpha(ch))
	        {
		    string_value = ch;
		    while (cin.get(ch) && isalnum(ch)) string_value.push_back(ch);
		    cin.putback(ch);
		    return curr_tok = NAME;
	        }
	        error("Bad Token");
	        return curr_tok = PRINT;
      }
}


double create()      //Dont mind this part, im trying to improve the calc
{                           //by ading the possibility to enter user functions..
      int number = 0;
      double ref = 0.0;
      string temp, name;
      cout << "Enter a name for the expression : " << endl;
      cin >> name;
      cout << "Enter the number of variables in expression :" << endl;
      cin >> number;
      for(int j = 0, i =1; j < number; j++, i++)
      {
	  cout << "Enter a name for variable #" << i<< endl;
	  cin >> temp;
	  cout << "Enter a value for that variable :" << endl;
	  cin >> ref;
	  table[temp] = ref;
      }
      cout << "Enter an expression :" << endl;
      cin >> temp;
      expression[name] = temp;
      curr_tok = PRINT;
      return 1;
}

double error(const string &msg)
{
      no_of_errors++;
      cerr << "error : " << msg << " at line : " << line_count.current() <<  endl;
      return 1;
}

double number_value;
cin >> number_value;

code was made by stroustrup btw..

Yup, there is a subtle genius to it.