69 lines
2.2 KiB
C++
69 lines
2.2 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::hash`.
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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_HASH_HPP
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#define BOOST_HANA_FWD_HASH_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Returns a `hana::type` representing the compile-time hash of an object.
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//! @ingroup group-Hashable
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//!
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//! Given an arbitrary object `x`, `hana::hash` returns a `hana::type`
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//! representing the hash of `x`. In normal programming, hashes are
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//! usually numerical values that can be used e.g. as indices in an
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//! array as part of the implementation of a hash table. In the context
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//! of metaprogramming, we are interested in type-level hashes instead.
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//! Thus, `hana::hash` must return a `hana::type` object instead of an
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//! integer. This `hana::type` must somehow summarize the object being
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//! hashed, but that summary may of course lose some information.
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//!
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//! In order for the `hash` function to be defined properly, it must be
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//! the case that whenever `x` is equal to `y`, then `hash(x)` is equal
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//! to `hash(y)`. This ensures that `hana::hash` is a function in the
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//! mathematical sense of the term.
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//!
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//!
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//! Signature
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//! ---------
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//! Given a `Hashable` `H`, the signature is
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//! \f$
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//! \mathtt{hash} : H \to \mathtt{type\_tag}
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//! \f$
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//!
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//! @param x
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//! An object whose hash is to be computed.
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//!
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//!
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//! Example
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//! -------
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//! @include example/hash.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto hash = [](auto const& x) {
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return tag-dispatched;
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};
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#else
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template <typename T, typename = void>
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struct hash_impl : hash_impl<T, when<true>> { };
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struct hash_t {
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template <typename X>
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constexpr auto operator()(X const& x) const;
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};
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constexpr hash_t hash{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_HASH_HPP
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