cpp/CPPlusV6/Chapter 15/error5.cpp

//error5.cpp -- unwinding the stack
#include <iostream>
#include <cmath> // or math.h, unix users may need -lm flag
#include <string>
#include "exc_mean.h"

class demo
{
private:
    std::string word;
public:
    demo (const std::string & str)
    {
        
        word = str;
        std::cout << "demo " << word << " created\n";
    }
    ~demo()
    {
        std::cout << "demo " << word << " destroyed\n";
    }
    void show() const
    {
        std::cout << "demo " << word << " lives!\n";
    }
}; 

// function prototypes
double hmean(double a, double b);
double gmean(double a, double b);
double means(double a, double b);

int main()
{
    using std::cout;
    using std::cin;
    using std::endl;
    
    double x, y, z;
	{
        demo d1("found in block in main()");
        cout << "Enter two numbers: ";
        while (cin >> x >> y)
        {
               try {                  // start of try block
                   z = means(x,y);
                   cout << "The mean mean of " << x << " and " << y
                           << " is " << z << endl;
                   cout << "Enter next pair: ";
               } // end of try block
               catch (bad_hmean & bg)    // start of catch block
               {
                   bg.mesg();
                   cout << "Try again.\n";
                   continue;
               }                  
               catch (bad_gmean & hg) 
               {
                   cout << hg.mesg();
                   cout << "Values used: " << hg.v1 << ", " 
                           << hg.v2 << endl;
                   cout << "Sorry, you don't get to play any more.\n";
                   break;
               } // end of catch block
        }
        d1.show();
    }
    cout << "Bye!\n";
    // cin.get();
    // cin.get();
    return 0;
}

double hmean(double a, double b)
{
    if (a == -b)
        throw bad_hmean(a,b);
    return 2.0 * a * b / (a + b);
}

double gmean(double a, double b)
{
    if (a < 0 || b < 0)
        throw bad_gmean(a,b);
    return std::sqrt(a * b); 
}

double means(double a, double b)
{
    double am, hm, gm;
    demo d2("found in means()");
    am = (a + b) / 2.0;    // arithmetic mean
    try 
    {
        hm = hmean(a,b);
        gm = gmean(a,b);
    }
    catch (bad_hmean & bg) // start of catch block
    {
        bg.mesg();
        std::cout << "Caught in means()\n";
        throw;             // rethrows the exception 
    }          
    d2.show();
    return (am + hm + gm) / 3.0;
}