Exception Demo

#include <iostream>
#include <stdexcept>
using namespace std;

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;
}

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

   while (quitCmd != 'q') {
      try {
         // Get user data
         weightVal = GetWeight();
         heightVal = GetHeight();
         
         // Calculate BMI and print user health info if no input error
         // Source: http://www.cdc.gov/
         bmiCalc = (static_cast<float>(weightVal) /
                    static_cast<float>(heightVal * heightVal)) * 703.0;
         
         cout << "BMI: " << bmiCalc << endl;
         cout << "(CDC: 18.6-24.9 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 >> quitCmd;
   }
   
   return 0;
}

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.