Coding⏱️ 2 min read📅 2026-05-31

How to Fix: C++ Exceptions questions on rethrow of original exception

The rethrown exception will reflect the call to append() in both cases, as it is passed by value and its contents are copied when thrown.

Quick Answer: In C++, exceptions are passed by value, so changes made to the exception object within the catch block are reflected in the rethrown exception.

When rethrowing an exception in C++, the behavior of the rethrown exception is determined by the type of the caught exception. If the caught exception is a reference to its base class, any modifications made to the derived class will not be reflected in the rethrown exception.

🔍 Why This Happens

  • When you call `throw err;` in the catch block, a new exception object is created and its constructor is called with the modified `err` reference. The original exception's destructor is not called until the new exception object is destroyed.

🔧 Proven Troubleshooting Steps

Method 1: Understanding Exception Inheritance

  1. Step 1: Study the C++ exception handling rules, particularly how exceptions are handled when derived classes are involved.

Method 2: Verifying Exceptions with a Debugger

  1. Step 1: Use a debugger to step through your code, particularly around the `throw err;` statement.

🎯 Final Words

To ensure that modifications made to a derived exception are reflected in the rethrown exception, use `std::rethrow_exception` with the original exception object. This approach guarantees that the correct exception is rethrown with its original state intact.

Did this fix your problem?

If not, try searching for specific error codes.

🔍 Search Error Database

❓ Frequently Asked Questions