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When to Catch Exceptions by Value or Reference: A Guide to Best Practices

In C++, there are two common ways to handle exceptions: catching by value and catching by reference. The question arises as to which approach is preferable in different scenarios.

According to the standard practice for exceptions in C++, the correct approach is to throw by value and catch by reference. Let's explore the reasons behind this recommendation:

Throw by Value:

• Ensures that the exception object is a copy of the original thrown object, preserving any data associated with it.
• Prevents unexpected modifications to the original exception object.

Catch by Reference:

• Avoids the overhead of copying the exception object, which can be significant for complex objects with large data structures.
• Allows for direct access and modification of the exception object within the catch block.

Catching by Value is Problematic with Inheritance Hierarchies:

Suppose you have an exception class CustomException and a derived class MyException that overrides certain properties or methods. When you catch the exception by value, as in the example below:

try {
  ...
} catch (CustomException e) {
  ...
}

If a MyException object is thrown, it will be converted to a CustomException instance upon being caught, potentially resulting in the loss of derived properties or behavior.

Example:

If MyException overrides the error_code member, catching by value would cause an unexpected change in the error code when a MyException object is thrown.

In Summary:

For most scenarios, the recommended practice is to throw exceptions by value and catch them by reference to avoid potential issues with inheritance hierarchies. This ensures the integrity of the exception data and allows for direct access to the exception object within the catch block.

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