Object Oriented Programming C++ Simple Calculator
  This is a beginner C++ OOP program.   # include  <iostream>  # include  <limits>  # include  <math.h>    # include  <cmath>  // for log function  using  namespace  std ;  class  calculator {   public:     double add(double a, double b){         return a+b;     }      double  subtract (double a, double b){         return a-b;     }      double  multiply (double a, double b){         if(a==0 && b==0){             cout<<"Error! Invalid operation!"<<endl;             return std::numeric_limits<double>:: quiet_NaN ();         }         return a*b;     }      double  divide (double a, double b){         if(b==0)         {             cout<<"Error! Cannot divide by 0!"<<endl;             return std::numeric_limits<double>:: quiet_NaN ();         }         return a/b;     }      double  power (double a, double b){         return pow(a,b);     }      double  loggy (double a) { //can't be 0 or less     ...
