Class exception. Using class to handle exceptions

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Contents

• 1. Class exception. General information
• 2. An example of generating a simple exception of the exception class
• 3. An example of developing a function that uses the exception class to generate an exception can occur if the quadratic equation has no solution
• Related topics

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1. Class exception. General information

The C++ Standard Library provides the exception class for handling exceptions. To use the features of the exception class, you need to include the header <exception>

#include <exception>

Сlass has the following description:

class exception
{
public:
  exception() throw();
  exception(const bad_exception& ob) throw();
  virtual ~exception() throw();
  exception &operator=(const exception& ob) throw();
  virtual const char* what() const throw();
};

The exception class is the base class for all exceptions defined by the C++ standard. Any exceptional situation is described by a certain class. The following is a list of important classes inherited from the exception class:

• class bad_alloc – generates an exception in case of an unsuccessful attempt to allocate memory using the new operator;
• class bad_typeid – generates an exception when the typeid operator fails;
• class bad_cast – generates an exception in case of unsuccessful dynamic type casting.

Each of the inherited classes has a what() function that returns a pointer to a string terminated by ‘\0’. It is through this function that you can get a string with an explanation of the reason for the occurrence of a particular exceptional situation.

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2. An example of generating a simple exception of the exception class

The example demonstrates artificially generating and catching an exception of type exception. According to a similar pattern, you can develop classes, functions that describe (implement) their own unique system of exceptions.

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

// Класс exception, пример генерирования простого исключения
void DemoException()
{
  try {
    std::exception err("My error"); // create an object
    throw err; // generate the exception
  }
  catch (std::exception& error)
  {
    std::cout << error.what() << endl;
  }
}

void main(void)
{
  DemoException();
  return;
}

Program result

My error

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3. An example of developing a function that uses the exception class to generate an exception can occur if the quadratic equation has no solution

A function is proposed that solves a quadratic equation based on the input coefficients a, b, c. As you know, for some values of a, b, c it is impossible to obtain a solution to the equation. For demonstration purposes, the function throws an exception if the quadratic equation has no roots. The exception class is used to create an exception.

#include <iostream>
#include <exception> // needed to use the exception class
using namespace std;

// An example of catching the exception of the absence of a solution
// to a quadratic equation in a function.
// Parameters:
// - a, b, c - equation coefficients a*x^2 + b*x + c = 0;
// - x1, x2 - solution of the equation.
// The function returns true if a solution exists,
// otherwise, the function returns false.
bool DemoSquareEquation(double a, double b, double c,
    double& x1, double& x2)
{
  // 1. Additional internal variables
  double D; // discriminant

  // 2. Calculate discriminant
  D = b * b - 4 * a * c;

  // 3. Processing the D value
  try
  {
    if (D < 0) // If the equation has no solution
    {
      // Then throw the exception
      // 3.1. Create an error object of type exception
      exception error("The equation has no roots");

      // 3.2. Throw the exception
      throw error;
    }
  }
  catch (exception& e)
  {
    // 3.3. Print the result
    cout << e.what() << endl;
    return false;
  }

  // 4. If the equation has a solution, then get and return the result
  x1 = (-b - sqrt(D)) / (2 * a);
  x2 = (-b + sqrt(D)) / (2 * a);

  // 5. Return the result
  return true;
}

void main(void)
{
  // 1. Declare internal variables
  double x1, x2;

  // 2. Invoke the DemoSquareEquation()
  if (DemoSquareEquation(2, 3, 4, x1, x2))
  {
    cout << "x1 = " << x1 << endl;
    cout << "x2 = " << x2 << endl;
  }

  return;
}

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Related topics

• The concept of exception. Instruction try-catch. Examples. Operator throw
• An example of creating a class hierarchy for exception handling

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