101 lines
3.2 KiB
C++
101 lines
3.2 KiB
C++
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/*!
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@file
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Forward declares `boost::hana::value`.
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@copyright Louis Dionne 2013-2016
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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_VALUE_HPP
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#define BOOST_HANA_FWD_VALUE_HPP
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/when.hpp>
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BOOST_HANA_NAMESPACE_BEGIN
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//! Return the compile-time value associated to a constant.
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//! @ingroup group-Constant
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//!
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//! This function returns the value associated to a `Constant`. That
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//! value is always a constant expression. The normal way of using
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//! `value` on an object `c` is
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//! @code
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//! constexpr auto result = hana::value<decltype(c)>();
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//! @endcode
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//!
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//! However, for convenience, an overload of `value` is provided so that
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//! it can be called as:
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//! @code
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//! constexpr auto result = hana::value(c);
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//! @endcode
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//!
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//! This overload works by taking a `const&` to its argument, and then
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//! forwarding to the first version of `value`. Since it does not use
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//! its argument, the result can still be a constant expression, even
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//! if the argument is not a constant expression.
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//!
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//! @note
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//! `value<T>()` is tag-dispatched as `value_impl<C>::%apply<T>()`, where
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//! `C` is the tag of `T`.
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//!
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//! @note
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//! `hana::value` is an overloaded function, not a function object.
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//! Hence, it can't be passed to higher-order algorithms. If you need
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//! an equivalent function object, use `hana::value_of` instead.
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//!
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//!
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//! Example
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//! -------
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//! @include example/value.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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template <typename T>
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constexpr auto value = []() -> decltype(auto) {
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return tag-dispatched;
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};
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#else
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template <typename C, typename = void>
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struct value_impl : value_impl<C, when<true>> { };
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template <typename T>
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constexpr decltype(auto) value();
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template <typename T>
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constexpr decltype(auto) value(T const&)
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{ return hana::value<T>(); }
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#endif
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//! Equivalent to `value`, but can be passed to higher-order algorithms.
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//! @ingroup group-Constant
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//!
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//! This function object is equivalent to `value`, except it can be passed
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//! to higher order algorithms because it is a function object. `value`
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//! can't be passed to higher-order algorithms because it is implemented
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//! as an overloaded function.
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//!
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//! @note
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//! This function is a simple alias to `value`, and hence it is not
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//! tag-dispatched and can't be customized.
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//!
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//!
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//! Example
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//! -------
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//! @include example/value_of.cpp
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#ifdef BOOST_HANA_DOXYGEN_INVOKED
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constexpr auto value_of = [](auto const& c) -> decltype(auto) {
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return hana::value(c);
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};
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#else
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struct value_of_t {
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template <typename T>
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constexpr decltype(auto) operator()(T const&) const
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{ return hana::value<T>(); }
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};
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constexpr value_of_t value_of{};
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#endif
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_FWD_VALUE_HPP
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