C++ Exceptions and Exception Handling: Interview Questions & Answers

Prepare for your C++ interviews! This resource covers the fundamentals of exceptions and exception handling in C++, providing answers to common interview questions. Learn how to handle runtime errors gracefully and write more robust C++ code.

Top C++ Exception Handling Interview Questions

Before diving into the questions and answers, let's briefly understand exceptions and exception handling in C++.

What is an Exception?

An exception is an unexpected event that disrupts the normal flow of a program. It's an error or failure condition that occurs during program execution.

Causes of Exceptions

  • Runtime Errors: These happen while the program runs, like dividing by zero, accessing incorrect memory, or performing illegal operations.
  • External Conditions: Problems outside your code, such as network issues, file access failures, or insufficient resources.
  • Invalid Data/Input: The program receives unexpected or wrong data, e.g., trying to convert a non-number string to a number.
  • Environmental Issues: Problems with the environment, such as a missing file, database connection failure, or hardware malfunction.

Exception Handling in C++

Exception handling in C++ provides a structured way to manage runtime errors. It prevents your program from crashing and allows for graceful error recovery.

Key Components of Exception Handling

  • try blocks: The code that might throw an exception is placed inside a try block.
  • catch blocks: These blocks handle exceptions thrown by the corresponding try block. Each catch block specifies the type of exception it can handle.
  • throw expression: This statement signals that an exception has occurred. It can be used to throw built-in types, user-defined types, or standard library exceptions.
Syntax: throw expression

throw std::runtime_error("Something went wrong!");

Interview Questions and Answers

1) What is exception handling in C++?

Exception handling is a mechanism in C++ for managing runtime errors or exceptional conditions that might occur during program execution. It allows you to catch and handle these errors gracefully.

2) Advantages of Exception Handling

  • Separates error-handling code from the main program logic, improving readability and maintainability.
  • Allows error propagation across different parts of the program.
  • Facilitates quick error recovery.
  • Enables centralized error handling.

3) How does exception handling work in C++?

It uses try, catch, and throw keywords.

  1. Error-prone code goes in a try block.
  2. If an error occurs, the throw keyword signals the exception.
  3. A matching catch block handles the exception.
Example

#include <iostream>
#include <exception>

int main() {
  try {
    int result = 10 / 0; // This will throw an exception
    std::cout << "Result: " << result << std::endl;
  } catch (const std::exception& e) {
    std::cerr << "Exception caught: " << e.what() << std::endl;
  }
  return 0;
}
Output

Exception caught: std::exception

4) Importance of the throw statement

The throw statement explicitly raises an exception. It's used when a specific condition requires immediate attention.

5) Importance of the try block

The try block contains code that might throw an exception. It's where exception monitoring starts.

6) Importance of the catch block

The catch block handles exceptions thrown in a corresponding try block. It specifies the exception type it handles and the code to execute when that exception occurs.

7) What happens if an exception is thrown but not caught?

The program terminates abnormally. The runtime system unwinds the call stack, cleans up, and ends the program.

8) Nesting try blocks

Yes, you can nest try blocks. This allows handling exceptions at different levels. Inner try blocks are checked first; if no match is found, the outer blocks are checked.

9) Checked vs. Unchecked Exceptions in C++

C++ doesn't directly distinguish between checked and unchecked exceptions like some other languages (e.g., Java). All exceptions can be caught if you provide appropriate catch blocks.

10) throw vs. throws keywords

throw raises an exception. throws (used in languages like Java) is a keyword that declares which exceptions a method might throw.

11) Handling Exceptions by Value, Reference, and Pointer

  • Value: Copies the exception object. Allows modification but has the overhead of copying.
  • Reference: Avoids copying, directly accessing the original object.
  • Pointer: Allows handling null pointers and flexibility in rethrowing or reassigning.

12) Role of Destructors in Exception Handling

Destructors are crucial. They clean up resources (like memory) when exceptions occur. When an exception is thrown, destructors for objects in the try block's scope are automatically called.

13) throw with and without arguments

  • Without argument: Rethrows the currently handled exception.
  • With argument: Throws a new exception of a specified type.

14) Rethrowing Exceptions

Use throw; (without an argument) inside a catch block to rethrow the current exception.

15) Stack Unwinding

Stack unwinding is the process of deallocating objects and calling destructors on the call stack when an exception is thrown. The runtime searches for a matching catch block during this unwinding process.

16) Standard vs. Custom Exceptions

  • Standard Exceptions: Predefined exceptions from the C++ Standard Library (e.g., std::runtime_error, std::invalid_argument).
  • Custom Exceptions: User-defined exceptions, often inheriting from std::exception. These are tailored to specific application needs.

17) Handling Exceptions in Constructors and Destructors

Use try-catch blocks within constructors. If an exception occurs in a destructor and isn't caught within the destructor, the program usually terminates.

18) Catching Exceptions by Reference

Yes, you can catch exceptions by reference (e.g., catch (const std::exception& e)) instead of by value. This is generally more efficient because it avoids copying the exception object.

18) Catching Exceptions by Reference

You can catch exceptions using a reference (e.g., catch (const std::exception& e)) instead of by value. This is generally preferred because it avoids the overhead of copying the potentially large exception object.

19) Exception Safety

Exception safety ensures your program maintains a consistent and valid state even if exceptions occur. This means properly managing resources, preventing memory leaks, and leaving objects in a usable condition, regardless of whether an exception is thrown.

20) Levels of Exception Safety

Exception safety is typically categorized into three levels:

No-throw Guarantee

Functions with a no-throw guarantee promise they will *never* throw an exception under any circumstances. This is the strongest level of exception safety.

Basic Exception Safety

Functions with basic exception safety guarantee that resources (like memory) won't be leaked and the program remains valid even if an exception is thrown. However, the function's intended operation might not be fully completed.

Strong Exception Safety

Functions offering strong exception safety guarantee that if an exception occurs, the program's state will be restored to what it was *before* the function was called. There are no memory leaks or unintended side effects. This is the highest level of exception safety but often the hardest to achieve.