////
Copyright 2003-2017 Beman Dawes

Distributed under the Boost Software License, Version 1.0.

See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt
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[#rationale]
# Design Rationale
:idprefix: rationale_

`error_code` and `error_condition` are designed as value types so
they can be copied without slicing and do not require heap allocation, but
still have polymorphic behavior based on the error category. This is achieved
by abstract base class `error_category` supplying the polymorphic behavior,
and `error_code` and `error_condition` containing a pointer to an object of a
type derived from `error_category`.

Many of the detailed design decisions were driven by the requirements that
users to be able to add additional error categories, and that it be no more
difficult to write portable code than system-specific code.

The `operator<<` overload for `error_code` eliminates a misleading conversion to
`bool` in code like `cout << ec`, where `ec` is of type `error_code`. It is also
useful in its own right.