hana/fwd/product.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::product`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_FWD_PRODUCT_HPP
0011 #define BOOST_HANA_FWD_PRODUCT_HPP
0012
0013 #include <boost/hana/config.hpp>
0014 #include <boost/hana/core/when.hpp>
0015 #include <boost/hana/fwd/integral_constant.hpp>
0016
0017
0018 namespace boost { namespace hana {
0019     //! Compute the product of the numbers of a structure.
0020     //! @ingroup group-Foldable
0021     //!
0022     //! More generally, `product` will take any foldable structure containing
0023     //! objects forming a Ring and reduce them using the Ring's binary
0024     //! operation. The initial state for folding is the identity of the
0025     //! Ring's operation. It is sometimes necessary to specify the Ring to
0026     //! use; this is possible by using `product<R>`. If no Ring is specified,
0027     //! the structure will use the Ring formed by the elements it contains
0028     //! (if it knows it), or `integral_constant_tag<int>` otherwise.
0029     //! Hence,
0030     //! @code
0031     //!     product<R>(xs) = fold_left(xs, one<R or inferred Ring>(), mult)
0032     //!     product<> = product<integral_constant_tag<int>>
0033     //! @endcode
0034     //!
0035     //! For numbers, this will just compute the product of the numbers in the
0036     //! `xs` structure.
0037     //!
0038     //! @note
0039     //! The elements of the structure are not actually required to be in the
0040     //! same Ring, but it must be possible to perform `mult` on any two
0041     //! adjacent elements of the structure, which requires each pair of
0042     //! adjacent element to at least have a common Ring embedding. The
0043     //! meaning of "adjacent" as used here is that two elements of the
0044     //! structure `x` and `y` are adjacent if and only if they are adjacent
0045     //! in the linearization of that structure, as documented by the Iterable
0046     //! concept.
0047     //!
0048     //! @note
0049     //! See the documentation for `sum` to understand why the Ring must
0050     //! sometimes be specified explicitly.
0051     //!
0052     //!
0053     //! Example
0054     //! -------
0055     //! @include example/product.cpp
0056 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0057     constexpr auto product = see documentation;
0058 #else
0059     template <typename T, typename = void>
0060     struct product_impl : product_impl<T, when<true>> { };
0061
0062     template <typename R>
0063     struct product_t {
0064         template <typename Xs>
0065         constexpr decltype(auto) operator()(Xs&& xs) const;
0066     };
0067
0068     template <typename R = integral_constant_tag<int>>
0069     BOOST_HANA_INLINE_VARIABLE constexpr product_t<R> product{};
0070 #endif
0071 }} // end namespace boost::hana
0072
0073 #endif // !BOOST_HANA_FWD_PRODUCT_HPP