1 /*! 2 @file 3 Forward declares `boost::hana::if_`. 4 5 @copyright Louis Dionne 2013-2017 6 Distributed under the Boost Software License, Version 1.0. 7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) 8 */ 9 10 #ifndef BOOST_HANA_FWD_IF_HPP 11 #define BOOST_HANA_FWD_IF_HPP 12 13 #include <boost/hana/config.hpp> 14 #include <boost/hana/core/when.hpp> 15 16 17 BOOST_HANA_NAMESPACE_BEGIN 18 //! Conditionally return one of two values based on a condition. 19 //! @ingroup group-Logical 20 //! 21 //! Specifically, `then` is returned iff `cond` is true-valued, and 22 //! `else_` is returned otherwise. Note that some `Logical` models may 23 //! allow `then` and `else_` to have different types, while others may 24 //! require both values to have the same type. 25 //! 26 //! 27 //! @param cond 28 //! The condition determining which of the two values is returned. 29 //! 30 //! @param then 31 //! The value returned when `cond` is true-valued. 32 //! 33 //! @param else_ 34 //! The value returned when `cond` is false-valued. 35 //! 36 //! 37 //! Example 38 //! ------- 39 //! @include example/if.cpp 40 #ifdef BOOST_HANA_DOXYGEN_INVOKED 41 constexpr auto if_ = [](auto&& cond, auto&& then, auto&& else_) -> decltype(auto) { 42 return tag-dispatched; 43 }; 44 #else 45 template <typename L, typename = void> 46 struct if_impl : if_impl<L, when<true>> { }; 47 48 struct if_t { 49 template <typename Cond, typename Then, typename Else> 50 constexpr decltype(auto) operator()(Cond&& cond, Then&& then, Else&& else_) const; 51 }; 52 53 constexpr if_t if_{}; 54 #endif 55 BOOST_HANA_NAMESPACE_END 56 57 #endif // !BOOST_HANA_FWD_IF_HPP 58