ceph/src/boost/boost/hana/fwd/hash.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::hash`.
   4
   5 @copyright Louis Dionne 2016
   6 @copyright Jason Rice 2016
   7 Distributed under the Boost Software License, Version 1.0.
   8 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   9  */
  10
  11 #ifndef BOOST_HANA_FWD_HASH_HPP
  12 #define BOOST_HANA_FWD_HASH_HPP
  13
  14 #include <boost/hana/config.hpp>
  15 #include <boost/hana/core/when.hpp>
  16
  17
  18 BOOST_HANA_NAMESPACE_BEGIN
  19     //! Returns a `hana::type` representing the compile-time hash of an object.
  20     //! @ingroup group-Hashable
  21     //!
  22     //! Given an arbitrary object `x`, `hana::hash` returns a `hana::type`
  23     //! representing the hash of `x`. In normal programming, hashes are
  24     //! usually numerical values that can be used e.g. as indices in an
  25     //! array as part of the implementation of a hash table. In the context
  26     //! of metaprogramming, we are interested in type-level hashes instead.
  27     //! Thus, `hana::hash` must return a `hana::type` object instead of an
  28     //! integer. This `hana::type` must somehow summarize the object being
  29     //! hashed, but that summary may of course lose some information.
  30     //!
  31     //! In order for the `hash` function to be defined properly, it must be
  32     //! the case that whenever `x` is equal to `y`, then `hash(x)` is equal
  33     //! to `hash(y)`. This ensures that `hana::hash` is a function in the
  34     //! mathematical sense of the term.
  35     //!
  36     //!
  37     //! Signature
  38     //! ---------
  39     //! Given a `Hashable` `H`, the signature is
  40     //! \f$
  41     //!     \mathtt{hash} : H \to \mathtt{type\_tag}
  42     //! \f$
  43     //!
  44     //! @param x
  45     //! An object whose hash is to be computed.
  46     //!
  47     //!
  48     //! Example
  49     //! -------
  50     //! @include example/hash.cpp
  51 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  52     constexpr auto hash = [](auto const& x) {
  53         return tag-dispatched;
  54     };
  55 #else
  56     template <typename T, typename = void>
  57     struct hash_impl : hash_impl<T, when<true>> { };
  58
  59     struct hash_t {
  60         template <typename X>
  61         constexpr auto operator()(X const& x) const;
  62     };
  63
  64     constexpr hash_t hash{};
  65 #endif
  66 BOOST_HANA_NAMESPACE_END
  67
  68 #endif // !BOOST_HANA_FWD_HASH_HPP