ceph/src/boost/libs/hana/include/boost/hana/fwd/product.hpp

   1 /*!
   2 @file
   3 Forward declares `boost::hana::product`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_FWD_PRODUCT_HPP
  11 #define BOOST_HANA_FWD_PRODUCT_HPP
  12
  13 #include <boost/hana/config.hpp>
  14 #include <boost/hana/core/when.hpp>
  15 #include <boost/hana/fwd/integral_constant.hpp>
  16
  17
  18 BOOST_HANA_NAMESPACE_BEGIN
  19     //! Compute the product of the numbers of a structure.
  20     //! @ingroup group-Foldable
  21     //!
  22     //! More generally, `product` will take any foldable structure containing
  23     //! objects forming a Ring and reduce them using the Ring's binary
  24     //! operation. The initial state for folding is the identity of the
  25     //! Ring's operation. It is sometimes necessary to specify the Ring to
  26     //! use; this is possible by using `product<R>`. If no Ring is specified,
  27     //! the structure will use the Ring formed by the elements it contains
  28     //! (if it knows it), or `integral_constant_tag<int>` otherwise.
  29     //! Hence,
  30     //! @code
  31     //!     product<R>(xs) = fold_left(xs, one<R or inferred Ring>(), mult)
  32     //!     product<> = product<integral_constant_tag<int>>
  33     //! @endcode
  34     //!
  35     //! For numbers, this will just compute the product of the numbers in the
  36     //! `xs` structure.
  37     //!
  38     //! @note
  39     //! The elements of the structure are not actually required to be in the
  40     //! same Ring, but it must be possible to perform `mult` on any two
  41     //! adjacent elements of the structure, which requires each pair of
  42     //! adjacent element to at least have a common Ring embedding. The
  43     //! meaning of "adjacent" as used here is that two elements of the
  44     //! structure `x` and `y` are adjacent if and only if they are adjacent
  45     //! in the linearization of that structure, as documented by the Iterable
  46     //! concept.
  47     //!
  48     //! @note
  49     //! See the documentation for `sum` to understand why the Ring must
  50     //! sometimes be specified explicitly.
  51     //!
  52     //!
  53     //! Example
  54     //! -------
  55     //! @include example/product.cpp
  56 #ifdef BOOST_HANA_DOXYGEN_INVOKED
  57     constexpr auto product = see documentation;
  58 #else
  59     template <typename T, typename = void>
  60     struct product_impl : product_impl<T, when<true>> { };
  61
  62     template <typename R>
  63     struct product_t;
  64
  65     template <typename R = integral_constant_tag<int>>
  66     constexpr product_t<R> product{};
  67 #endif
  68 BOOST_HANA_NAMESPACE_END
  69
  70 #endif // !BOOST_HANA_FWD_PRODUCT_HPP