Exception Demo

An exception is an error event that happens during the execution of a program, and it disrupts the normal flow of the program's instructions. When an error occurs, the program throws an exception, which must be caught and handled in some way. If not handled, the program usually crashes.

In C++, exception handling involves three keywords: try, catch, and throw.

try: This block contains any code that might throw an exception. The try keyword is followed by a block of code within braces {}.
throw: When an error condition occurs within a try block, the program throws an exception by using the throw keyword followed by an exception value. This value can be any of the built-in data type values or an object of a user-defined exception class.
catch: This block is used to handle the exception. It follows the try block and catches an exception thrown within the try block. The type of exception it can catch must be specified inside parentheses following the catch keyword. If the type matches the exception thrown, this block will execute.

//  main.cpp
//  ExceptionDemoTwo//
//  Created by Kevin Roark on 6/30/23.
// Exception Demo - This C++ program is designed to calculate the
// Body Mass Index (BMI) of a user based on their inputted
// weight and height. The program demonstrates exception handling in C++.


#include <iostream>
#include <stdexcept> // Needed Libray for Exceptions
using namespace std;

//Prototypes
int GetWeight();
int GetHeight();

int main() {
   int weightVal;        // User defined weight (lbs)
   int heightVal;        // User defined height (in)
   double bmiCalc;        // Resulting BMI
   char quitCommand;         // Indicates quit/continue
   
   quitCommand = 'a';

   while (quitCommand != 'q') {
      try {
         // Get user data
         weightVal = GetWeight();
         heightVal = GetHeight();
         
         // Calculate BMI and print user health info if no input error
         bmiCalc = (static_cast<double>(weightVal) /
                    static_cast<double>(heightVal * heightVal)) * 703.0;
         
         cout << "BMI: " << bmiCalc << endl;
         cout << "(CDC: 18.6-24.9 is normal)" << endl;
      }
      catch (runtime_error &excpt) {
         // Prints the error message passed by throw statement
         cout << excpt.what() << endl;
         cout << "Cannot compute health info." << endl;
      }
      
      // Prompt user to continue/quit
      cout << endl << "Enter any key ('q' to quit): ";
      cin >> quitCommand;
   }
   
   return 0;
}

//Functions ************
int GetWeight() {
   int weightParam;     // User defined weight
   
   // Get user data
   cout << "Enter weight (in pounds): ";
   cin >> weightParam;
   
   // Error checking, non-negative weight
   if (weightParam < 0) {
      throw runtime_error("Invalid weight.");
   }
   return weightParam;
}

int GetHeight() {
   int heightParam;     // User defined height
   
   // Get user data
   cout << "Enter height (in inches): ";
   cin >> heightParam;
   
   // Error checking, non-negative height
   if (heightParam < 0) {
      throw runtime_error("Invalid height.");
   }
   return heightParam;
}

Suppose getWeight() throws an exception of type Exception. GetWeight() immediately exits, up to main() where the call was in a try block, so the catch block catches the exception.

Note the clarity of the code in main(). Without exceptions, GetWeight() would have had to somehow indicate failure, perhaps returning -1. Then main() would have needed an if-else statement to detect such failure, obscuring the normal code.

If no handler is found going up the call hierarchy, then terminate() is called, which typically aborts the program.

This Example is designed to calculate a user's Body Mass Index (BMI) based on their inputted weight and height. The program demonstrates exception handling in C++. Here is a breakdown of the key parts:

Main function: The program's execution begins here. It is designed to keep asking the user for their weight and height until the user enters 'q' to quit. For each iteration, the program attempts to calculate and print the BMI. It does so inside a try block, where it calls GetWeight() and GetHeight() functions, calculates the BMI, and prints it. If the user enters a negative value for either weight or height, an exception is thrown in either the GetWeight() or GetHeight() function. This exception is caught in the catch block in the main function, and an appropriate error message is printed.
GetWeight() function: This function asks the user for their weight, stores it in weightParam and returns it. If the weight entered is negative, it throws a runtime error with the message "Invalid weight."
GetHeight() function: This function works similarly to the GetWeight() function. It asks the user for their height, stores it in heightParam and returns it. If the height entered is negative, it throws a runtime error with the message "Invalid height."
Error Handling: The program uses exception handling to handle cases where the user may enter a negative value for either weight or height. If either GetWeight() or GetHeight() functions encounter a negative value, they throw a runtime_error exception. This exception is caught in the catch block in the main function where an error message is printed.
BMI Calculation: The Body Mass Index (BMI) is calculated using the formula (weight / (height * height)) * 703.0, where weight is in pounds and height is in inches. The result is a measure of body fat based on weight and height that applies to adult men and women.

This is an example of how exception handling in C++ can be used to handle errors gracefully and provide informative error messages to the user.