71 lines
2.4 KiB
C++
71 lines
2.4 KiB
C++
/*!
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@file
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Forward declares `boost::hana::make`.
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Copyright Louis Dionne 2013-2022
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_FWD_CORE_MAKE_HPP
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#define BOOST_HANA_FWD_CORE_MAKE_HPP
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#include <boost/hana/config.hpp>
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namespace boost { namespace hana {
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//! @ingroup group-core
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//! Create an object of the given tag with the given arguments.
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//!
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//! This function serves the same purpose as constructors in usual C++.
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//! However, instead of creating an object of a specific C++ type, it
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//! creates an object of a specific tag, regardless of the C++ type
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//! of that object.
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//!
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//! This function is actually a variable template, so `make<T>` can be
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//! passed around as a function object creating an object of tag `T`.
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//! Also, it uses tag-dispatching so this is how it should be customized
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//! for user-defined tags.
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//!
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//! Finally, the default implementation of `make` is equivalent to calling
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//! the constructor of the given tag with the corresponding arguments.
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//! In other words, by default,
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//! @code
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//! make<T>(args...) == T(args...)
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//! @endcode
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//!
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//! Note that the arguments are perfectly forwarded and the form of
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//! construction which is used is exactly as documented, i.e. `T(args...)`.
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//! However, if `T(args...)` is not a valid expression, a compilation
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//! error is triggered. This default behavior is useful because it makes
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//! foreign C++ types that have no notion of tag constructible with `make`
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//! out-of-the-box, since their tag is exactly themselves.
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//!
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//!
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//! Example
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//! -------
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//! @include example/core/make.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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template <typename Tag>
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constexpr auto make = [](auto&& ...x) -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename Tag, typename = void>
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struct make_impl;
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template <typename Tag>
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struct make_t {
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template <typename ...X>
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constexpr decltype(auto) operator()(X&& ...x) const {
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return make_impl<Tag>::apply(static_cast<X&&>(x)...);
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}
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};
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template <typename Tag>
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BOOST_HANA_INLINE_VARIABLE constexpr make_t<Tag> make{};
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#endif
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}} // end namespace boost::hana
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#endif // !BOOST_HANA_FWD_CORE_MAKE_HPP
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