fwd/core/is_a.hpp

0001 /*!
0002 @file
0003 Forward declares `boost::hana::is_a` and `boost::hana::is_an`.
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_CORE_IS_A_HPP
0011 #define BOOST_HANA_FWD_CORE_IS_A_HPP
0012
0013 #include <boost/hana/config.hpp>
0014
0015
0016 namespace boost { namespace hana {
0017     //! @ingroup group-core
0018     //! Returns whether the tag of an object matches a given tag.
0019     //!
0020     //! Given a tag `Tag` and a C++ type `T`, `is_a<Tag, T>` is a compile-time
0021     //! Logical representing whether the tag of `T` is exactly `Tag`. In other
0022     //! words, it is equivalent to
0023     //! @code
0024     //!     std::is_same<Tag, tag_of<T>::type>
0025     //! @endcode
0026     //!
0027     //! For convenience, an alternate syntax is provided for using `is_a`.
0028     //! Specifically, `is_a<Tag>` is a function object returning whether the
0029     //! argument it is passed has the given tag. In other words,
0030     //! @code
0031     //!     is_a<Tag>(x) == is_a<Tag, decltype(x)>
0032     //! @endcode
0033     //!
0034     //!
0035     //! Example
0036     //! -------
0037     //! @include example/core/is_a.cpp
0038 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0039     template <typename Tag, typename optional_T>
0040     constexpr auto is_a = see-documentation;
0041 #else
0042     template <typename Tag, typename ...T>
0043     struct is_a_t;
0044
0045     template <typename Tag, typename ...T>
0046     BOOST_HANA_INLINE_VARIABLE constexpr is_a_t<Tag, T...> is_a{};
0047 #endif
0048
0049     //! @ingroup group-core
0050     //! Equivalent to `is_a`; provided for consistency with the rules of the
0051     //! English language.
0052 #ifdef BOOST_HANA_DOXYGEN_INVOKED
0053     template <typename Tag, typename ...T>
0054     constexpr auto is_an = is_a<Tag, T...>;
0055 #else
0056     template <typename Tag, typename ...T>
0057     BOOST_HANA_INLINE_VARIABLE constexpr is_a_t<Tag, T...> is_an{};
0058 #endif
0059 }} // end namespace boost::hana
0060
0061 #endif // !BOOST_HANA_FWD_CORE_IS_A_HPP