hana/fwd/hash.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::hash`.
0004
0005 Copyright Louis Dionne 2016
0006 Copyright Jason Rice 2016
0007 Distributed under the Boost Software License, Version 1.0.
0008 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0009  */
0010
0011 #ifndef BOOST_HANA_FWD_HASH_HPP
0012 #define BOOST_HANA_FWD_HASH_HPP
0013
0014 #include <boost/hana/config.hpp>
0015 #include <boost/hana/core/when.hpp>
0016
0017
0018 namespace boost { namespace hana {
0019     //! Returns a `hana::type` representing the compile-time hash of an object.
0020     //! @ingroup group-Hashable
0021     //!
0022     //! Given an arbitrary object `x`, `hana::hash` returns a `hana::type`
0023     //! representing the hash of `x`. In normal programming, hashes are
0024     //! usually numerical values that can be used e.g. as indices in an
0025     //! array as part of the implementation of a hash table. In the context
0026     //! of metaprogramming, we are interested in type-level hashes instead.
0027     //! Thus, `hana::hash` must return a `hana::type` object instead of an
0028     //! integer. This `hana::type` must somehow summarize the object being
0029     //! hashed, but that summary may of course lose some information.
0030     //!
0031     //! In order for the `hash` function to be defined properly, it must be
0032     //! the case that whenever `x` is equal to `y`, then `hash(x)` is equal
0033     //! to `hash(y)`. This ensures that `hana::hash` is a function in the
0034     //! mathematical sense of the term.
0035     //!
0036     //!
0037     //! Signature
0038     //! ---------
0039     //! Given a `Hashable` `H`, the signature is
0040     //! \f$
0041     //!     \mathtt{hash} : H \to \mathtt{type\_tag}
0042     //! \f$
0043     //!
0044     //! @param x
0045     //! An object whose hash is to be computed.
0046     //!
0047     //!
0048     //! Example
0049     //! -------
0050     //! @include example/hash.cpp
0051 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0052     constexpr auto hash = [](auto const& x) {
0053         return tag-dispatched;
0054     };
0055 #else
0056     template <typename T, typename = void>
0057     struct hash_impl : hash_impl<T, when<true>> { };
0058
0059     struct hash_t {
0060         template <typename X>
0061         constexpr auto operator()(X const& x) const;
0062     };
0063
0064     BOOST_HANA_INLINE_VARIABLE constexpr hash_t hash{};
0065 #endif
0066 }} // end namespace boost::hana
0067
0068 #endif // !BOOST_HANA_FWD_HASH_HPP