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How to write a custom exception class in C++

Behzad Ahmad

Overview

The standard library of C++ provides users with the base class, which is specifically designed to declare the objects that used to be thrown as exceptions. The base class is defined in the <exception> header file and is called as std:exception. This class contains a virtual member method, which produces a null-terminated character sequence (of type char *) that may be rewritten in derived classes to provide an exception explanation.

Example

Let’s look at a basic example to better understand this concept.

#include <iostream> 
#include <exception> 
using namespace std; 
 
class myexception: public exception 
{ 
  virtual const char* what() const throw() 
  { 
    return "Custom Exception"; 
  } 
}; 
 
int main () 
{ 
  myexception ex; 
  try 
  { 
    throw ex; 
  } 
  catch (exception& except) 
  { 
    cout << except.what() << endl; 
  } 
  return 0; 
}
The custom exception class in C++

Explanation

  • Lines 1–3: We import the header files.

  • Lines 5–11: We make a user-defined exception class and inherit it from the except class. We use the virtual function to overload the what() function and return the exception.

  • Line 13: We write the main driver for the program.

  • Line 15: We make the object of the myexception class.

  • Lines 16–23: We use the try and catch statements to throw and then catch the statements, so that we can display the written message.

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