In C++ exception handling, when is it appropriate to use exceptions instead of returning simple error codes from a function?

Introduction / Context:
Exceptions in C++ provide a structured way to signal and handle error conditions that prevent a function from completing its intended work. Using exceptions appropriately helps keep normal code clean while centralising error handling. This question asks when it is suitable to use exceptions rather than returning error codes directly from a function.

Given Data / Assumptions:

• We are working in C++ with try, catch and throw constructs available.
• Functions may have contracts or postconditions that describe what they promise to do on success.
• Sometimes those contracts cannot be met due to problems such as missing resources or invalid input.

Concept / Approach:
The general guideline is to use exceptions for exceptional conditions, not for routine control flow. An exceptional condition is one in which a function cannot meet its specification, and simply returning a normal value would not be expressive enough or would clutter the interface. Exceptions allow you to separate normal code from error handling code and to propagate errors up the call stack until a suitable handler is found. For simple or very common conditions, error codes or optional return types may still be appropriate, but for serious violations of a function contract, exceptions are a good fit.

Step-by-Step Solution:
Step 1: Focus on the idea that exceptions are used when a function cannot complete its promised work normally.Step 2: Recognise that using exceptions for every branch of logic would make the code harder to read and may be slower.Step 3: Option A states that exceptions are used when postconditions cannot be satisfied and normal return values are not a clean way to represent the problem.Step 4: Options B, C, D and E either exaggerate the performance benefits or restrict exceptions to cases that do not match best practices.Step 5: Therefore, option A is the correct answer.

Verification / Alternative check:
Consider a function that opens a file and reads its contents. If the file does not exist or the permissions are wrong, the function cannot fulfil its contract of returning valid data. If you use exceptions, you can write straightforward code for the success path, and throw an exception when opening fails. The caller can then handle all file related errors in one place. This design reflects the idea in option A that exceptions signal failure to meet the function contract.

Why Other Options Are Wrong:
Option B incorrectly claims that exceptions automatically make normal control flow faster, which is not guaranteed and often not the primary reason for using them. Option C and option E are too narrow and ignore many valid use cases. Option D suggests using exceptions for normal business logic, which leads to confusing control flow and poor performance.

Common Pitfalls:
One common mistake is to overuse exceptions for control flow, such as throwing an exception for every minor validation error. Another pitfall is to ignore exceptions entirely and rely only on error codes, which can result in repetitive checks and missed error handling. Good C++ style uses exceptions for significant contract failures and keeps normal code paths clean and readable.

Final Answer:
When the function cannot satisfy its postconditions or contract and a normal return value cannot represent the error cleanly.