Note #4 - Robustness

Validate user input and directory specifications:

The two most common causes of run-time failures are incorrect user input and invalid directory specifications.
Designing for user input:
- Structure required input in terms the user will find easy to understand, e.g., labeled and consistent with the application's metaphor.
- Parse the input for errors and request re-entry if invalid, explaining carefully how to supply correct input. If your design of user input is hard to parse and validate then you need to re-think and revise the design. Often use of regular expression matching makes validation relatively painless. You will find a regular expression parser in the Boost libraries - www.boost.org.
Directory specifications and file handling are another frequent source of run-time errors:
- A directory path may not exist on the machine running the application.
- A file may not exist or be misidentified or be opened exclusively by another stream.
- File reads may have reached the end of the file.
Often the package encountering the error does not have information to rectify. In that case you should throw an exception with a meaningful message, expecting a higer-level package in your design - perhaps the Executive - to intercept and reinitialize.
You should also, in your manual information, warn readers that this exception may be thrown so they can write appropriate handlers.

Conclusions:

Programs are robust, not because they never encounter errors, but because they implement processing for validation and error handling.

Always validate user input and provide help for the user to correctly supply input.
Validate directory and file operations by wrapping stream operations in a try block. Consider rethrowing meaningful exception messages when errors occur.