User-defined Custom Exception with class in C++

In C++, exceptions are not limited to built-in data types. We can also define our own exception types using classes and throw objects of those classes. This allows developers to represent application-specific error conditions in a structured and type-safe way.

A user-defined exception is thrown using:

throw ClassName();

Example 1: Exception Handling with a Single Class

Caught exception of demo class

Explanation:

• demo is an empty user-defined class.
• An object of demo is thrown using throw demo().
• The corresponding catch block catches the exception based on its type.
• The exception is handled successfully without program termination.

Example 2: Exception Handling with Multiple Classes

Caught exception of demo1 class
Caught exception of demo2 class

Exception handling with inheritance

When exception classes are related through inheritance, the order of catch blocks becomes important. 

Caught exception of demo1 class
Caught exception of demo1 class

Explanation:

• demo2 is derived from demo1.
• The catch (demo1) block appears before catch (demo2).
• Since base-class handlers match derived objects, the demo2 exception is caught by demo1.
• Best practice: Always place derived-class catch blocks before base-class handlers.

Exception handling with constructor

Exceptions can also be thrown and handled inside constructors.

Exception caught
Zero not allowed
Again creating object
Num = 1

Explanation:

• The constructor validates input before initializing the object.
• When x == 0, an exception is thrown and caught inside the constructor.
• Object creation continues normally for valid input (x = 1).
• Constructors cannot return values, but exceptions allow error signaling during object creation.

Key Points to Remember

• C++ allows throwing objects of user-defined classes as exceptions.
• Exception handling is type-based, enabling fine-grained control.
• In inheritance-based exceptions, catch order matters.
• Constructors can throw and handle exceptions for input validation.
• User-defined exceptions improve clarity, maintainability, and robustness.