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 Hu Chunming
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/*!
@file
Forward declares `boost::hana::if_`.
Copyright Louis Dionne 2013-2022
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_IF_HPP
#define BOOST_HANA_FWD_IF_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
namespace boost { namespace hana {
//! Conditionally return one of two values based on a condition.
//! @ingroup group-Logical
//!
//! Specifically, `then` is returned iff `cond` is true-valued, and
//! `else_` is returned otherwise. Note that some `Logical` models may
//! allow `then` and `else_` to have different types, while others may
//! require both values to have the same type.
//!
//!
//! @param cond
//! The condition determining which of the two values is returned.
//!
//! @param then
//! The value returned when `cond` is true-valued.
//!
//! @param else_
//! The value returned when `cond` is false-valued.
//!
//!
//! Example
//! -------
//! @include example/if.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto if_ = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) {
return tag-dispatched;
};
#else
template <typename L, typename = void>
struct if_impl : if_impl<L, when<true>> { };
struct if_t {
template <typename Cond, typename Then, typename Else>
constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const;
};
BOOST_HANA_INLINE_VARIABLE constexpr if_t if_{};
#endif
}} // end namespace boost::hana
#endif // !BOOST_HANA_FWD_IF_HPP
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