exception types as simple semantic tags

# Boost Exception

### Exception Types as Simple Semantic Tags

Deriving from boost::exception effectively decouples the semantics of a failure from the information that is relevant to each individual instance of reporting a failure with a given semantic.

In other words: with boost::exception, what data a given exception object transports depends primarily on the context in which failures are reported (not on its type.) Since exception types need no members, it becomes very natural to throw exceptions that derive from more than one type to indicate multiple appropriate semantics:

    struct exception_base: virtual std::exception, virtual boost::exception { };
    struct io_error: virtual exception_base { };
    struct file_error: virtual io_error { };
    struct read_error: virtual io_error { };
    struct file_read_error: virtual file_error, virtual read_error { };

Using this approach, exception types become a simple tagging system for categorizing errors and selecting failures in exception handlers.

See also: Boost Exception | Motivation | Using Virtual Inheritance in Exception Types