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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::hash`.
Copyright Louis Dionne 2016
Copyright Jason Rice 2016
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_HASH_HPP
#define BOOST_HANA_FWD_HASH_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Returns a `hana::type` representing the compile-time hash of an object.
//! @ingroup group-Hashable
//!
//! Given an arbitrary object `x`, `hana::hash` returns a `hana::type`
//! representing the hash of `x`. In normal programming, hashes are
//! usually numerical values that can be used e.g. as indices in an
//! array as part of the implementation of a hash table. In the context
//! of metaprogramming, we are interested in type-level hashes instead.
//! Thus, `hana::hash` must return a `hana::type` object instead of an
//! integer. This `hana::type` must somehow summarize the object being
//! hashed, but that summary may of course lose some information.
//!
//! In order for the `hash` function to be defined properly, it must be
//! the case that whenever `x` is equal to `y`, then `hash(x)` is equal
//! to `hash(y)`. This ensures that `hana::hash` is a function in the
//! mathematical sense of the term.
//!
//!
//! Signature
//! ---------
//! Given a `Hashable` `H`, the signature is
//! \f$
//! \mathtt{hash} : H \to \mathtt{type\_tag}
//! \f$
//!
//! @param x
//! An object whose hash is to be computed.
//!
//!
//! Example
//! -------
//! @include example/hash.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto hash = [](auto const& x) {
return tag-dispatched;
};
#else
template <typename T, typename = void>
struct hash_impl : hash_impl<T, when<true>> { };
struct hash_t {
template <typename X>
constexpr auto operator()(X const& x) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr hash_t hash{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_HASH_HPP
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