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Diffstat (limited to 'boost/hana/fwd/core/common.hpp')
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diff --git a/boost/hana/fwd/core/common.hpp b/boost/hana/fwd/core/common.hpp new file mode 100644 index 0000000000..29702f23d5 --- /dev/null +++ b/boost/hana/fwd/core/common.hpp @@ -0,0 +1,103 @@ +/*! +@file +Forward declares `boost::hana::common` and `boost::hana::common_t`. + +@copyright Louis Dionne 2013-2016 +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_CORE_COMMON_HPP +#define BOOST_HANA_FWD_CORE_COMMON_HPP + +#include <boost/hana/config.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! @ingroup group-core + //! %Metafunction returning the common data type between two data types. + //! + //! `common` is a natural extension of the `std::common_type` metafunction + //! to data types. Given two data types `T` and `U`, we say that they share + //! a common type `C` if both objects of data type `T` and objects of data + //! type `U` may be converted (using `to`) to an object of data type `C`, + //! and if that conversion is equality preserving. In other words, this + //! means that for any objects `t1, t2` of data type `T` and `u1, u2` of + //! data type `U`, the following law is satisfied: + //! @code + //! to<C>(t1) == to<C>(t2) if and only if t1 == t2 + //! to<C>(u1) == to<C>(u2) if and only if u1 == u2 + //! @endcode + //! + //! The role of `common` is to provide an alias to such a `C` if it exists. + //! In other words, if `T` and `U` have a common data type `C`, + //! `common<T, U>::%type` is an alias to `C`. Otherwise, `common<T, U>` + //! has no nested `type` and can be used in dependent contexts to exploit + //! SFINAE. By default, the exact steps followed by `common` to determine + //! the common type `C` of `T` and `U` are + //! 1. If `T` and `U` are the same, then `C` is `T`. + //! 2. Otherwise, if `true ? std::declval<T>() : std::declval<U>()` is + //! well-formed, then `C` is the type of this expression after using + //! `std::decay` on it. This is exactly the type that would have been + //! returned by `std::common_type`, except that custom specializations + //! of `std::common_type` are not taken into account. + //! 3. Otherwise, no common data type is detected and `common<T, U>` does + //! not have a nested `type` alias, unless it is specialized explicitly. + //! + //! As point 3 suggests, it is also possible (and sometimes necessary) to + //! specialize `common` in the `boost::hana` namespace for pairs of custom + //! data types when the default behavior of `common` is not sufficient. + //! Note that `when`-based specialization is supported when specializing + //! `common` in the `boost::hana` namespace. + //! + //! > #### Rationale for requiring the conversion to be equality-preserving + //! > This decision is aligned with a proposed concept design for the + //! > standard library ([N3351][1]). Also, if we did not require this, + //! > then all data types would trivially share the common data type + //! > `void`, since all objects can be converted to it. + //! + //! + //! Example + //! ------- + //! @include example/core/common/common.cpp + //! + //! + //! [1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3351.pdf +#ifdef BOOST_HANA_DOXYGEN_INVOKED + template <typename T, typename U, optional when-based enabler> + struct common { see documentation }; +#else + template <typename T, typename U, typename = void> + struct common; +#endif + + //! @ingroup group-core + //! %Metafunction returning whether two data types share a common data type. + //! + //! Given two data types `T` and `U`, this metafunction simply returns + //! whether `common<T, U>::%type` is well-formed. + //! + //! + //! Example + //! ------- + //! @include example/core/common/has_common.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + template <typename T, typename U> + struct has_common { whether common<T, U>::type is well-formed }; +#else + template <typename T, typename U, typename = void> + struct has_common; +#endif + + //! @ingroup group-core + //! Alias to `common<T, U>::%type`, provided for convenience. + //! + //! + //! Example + //! ------- + //! @include example/core/common/common_t.cpp + template <typename T, typename U> + using common_t = typename common<T, U>::type; +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_CORE_COMMON_HPP |