#ifndef CalcEng_SEEN__
#define CalcEng_SEEN__
///////    File CalcEng.h    ///////
#define NULL 0

class CalcEng
{   public:
        CalcEng();
        virtual void operate(char c);    // perform operation c
        virtual char operation() { return op; }
        virtual void operand(double in);
        virtual double output()
        { return( argcnt == 2 ? arg : ans ); }
        static const char* keys;         // op keys recognized
        enum {PREC=15};                  // max guranteed precision
        // error indication by void* conversion
        operator void*()
        {  return(err_flag ?  NULL : this); }

    protected:
        virtual void compute();
        virtual void allclear();
        double ans, arg;
        int argcnt;
        char op;
        int err_flag;
};
#endif