69 lines
2.6 KiB
C++
69 lines
2.6 KiB
C++
/*!
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@file
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Forward declares `boost::hana::Hashable`.
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@copyright Louis Dionne 2016
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@copyright Jason Rice 2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_CONCEPT_HASHABLE_HPP
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#define BOOST_HANA_FWD_CONCEPT_HASHABLE_HPP
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#include <boost/hana/config.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @ingroup group-concepts
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//! @defgroup group-Hashable Hashable
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//! The `Hashable` concept represents objects that can be normalized to
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//! a type-level hash.
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//!
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//! In day to day programming, hashes are very important as a way to
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//! efficiently lookup objects in maps. While the implementation of
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//! maps is very different, the same idea of using hashes for efficient
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//! lookup applies in metaprogramming. The `Hashable` concept represents
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//! objects that can be summarized (possibly with loss of information) to
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//! a type, in a way suitable for use in hash-based data structures. Of
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//! course, in order for a hash to be well-behaved, it must obey some laws
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//! that are explained below.
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//!
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//!
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//! Minimal complete definition
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//! ---------------------------
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//! `hash`, satisfying the laws below
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//!
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//!
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//! Laws
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//! ----
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//! First, `hana::hash` must return a `hana::type`. Furthermore, for any
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//! two `Hashable` objects `x` and `y`, it must be the case that
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//! @code
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//! x == y implies hash(x) == hash(y)
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//! @endcode
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//!
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//! where `==` denotes `hana::equal`. In other words, any two objects that
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//! compare equal (with `hana::equal`) must also have the same hash.
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//! However, the reverse is not true, and two different objects may have
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//! the same hash. This situation of two different objects having the same
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//! hash is called a _collision_.
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//!
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//!
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//! Concrete models
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//! ---------------
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//! `hana::integral_constant`, `hana::type`, `hana::string`
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//!
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//!
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//! Free model for `IntegralConstant`s
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//! ----------------------------------
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//! Any `IntegralConstant` is `Hashable`, by normalizing its value to a
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//! `hana::integral_constant`. The type of the value held in the normalized
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//! `integral_constant` is `unsigned long long` for unsigned integral
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//! types, and `signed long long` for signed integral types.
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template <typename T>
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struct Hashable;
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_CONCEPT_HASHABLE_HPP
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