calc.cpp

BigC++的源码

#include <stack>
#include <iostream>
#include <stdexcept>

using namespace std;

/**
  A class that implements a simple RPN calculator.
*/
class Calculator 
{
public:
   enum BinaryOperator {PLUS, MINUS, TIMES, DIVIDE};

   /**
      Return the current top of stack, or throw
      exception if there is no current value.
      @return current top of stack
   */
   int current_memory() const;
  
   /**
      Push integer value onto top of stack.
      @param value integer to be placed on stack
   */
   void push_operand(int value);

   /**
      Perform indicated binary operation.
      @param op operation to be performed
   */
   void do_operator(BinaryOperator op);

private:
   stack<int> data;
};

/**
   A class that represents an input error.
*/
class CalcInputException : public runtime_error
{
public:
   CalcInputException(string why) : runtime_error(why) {}
};

int Calculator::current_memory() const
{
   if (data.empty())
      throw CalcInputException("Stack Underflow");
   return data.top();
}

inline void Calculator::push_operand(int value)
{
   data.push(value);
}

void Calculator::do_operator(BinaryOperator op)
{
   int right = current_memory();
   data.pop();
   int left = current_memory();
   data.pop();
   if (op == PLUS) data.push(left + right);
   else if (op == MINUS) data.push(left - right);
   else if (op == TIMES) data.push(left * right);
   else if (op == DIVIDE) data.push(left / right);   
}

/**
   Read from input stream, performing operations.
   Input consists of numbers, binary operators, or commands.
   command p - print
   command q - quit 
   @param in input stream to be read
*/
void read(istream& in) 
{
   Calculator calc;
   char c;

   while (in >> c)
      if (isdigit(c)) 
      {
         int intval;
         in.putback(c);
         in >> intval;
         calc.push_operand(intval);
      }
      else if (c == '+') 
         calc.do_operator(Calculator::PLUS); 
      else if (c == '-') 
         calc.do_operator(Calculator::MINUS); 
      else if (c == '*') 
         calc.do_operator(Calculator::TIMES); 
      else if (c == '/') 
         calc.do_operator(Calculator::DIVIDE); 
      else if (c == 'p') 
         cout << calc.current_memory() << "\n";
      else if (c == 'q')
         return;
}

int main() 
{
   try 
   {
      read(cin);
   } 
   catch (exception& e) 
   {
      cout << "caught exception " << e.what() << "\n";
   }
}