C++ Coding Standards: 101 Rules, Guidelines, and Best Practices

Stop errors at the boundary that has decision-making authority

Items 73–74, book pages 144–145; combined with Item 25, book page 46.

Discovering, translating, and handling errors are three distinct responsibilities. A parser knows which part of the input is syntactically invalid, but may not know whether to retry, prompt the user, or cancel the entire task; therefore it must not fake success just to return a number. If an upper layer is only responsible for forwarding, let the exception continue to propagate; only the place that has business context should add business meaning, and only the place that can choose a recovery action should stop the propagation. Catching by reference and a bare throw preserve the exception’s dynamic type, rather than copying and slicing information just so every layer has a catch. Modern code can also express failure with error values, but the boundary must make success and failure mutually exclusive; logging output must not be mistaken for recovery. The original example here uses parsing an amount to illustrate this division of responsibilities and does not copy the book’s cases.