ceph/src/boost/boost/pfr/detail/unsafe_declval.hpp

   1 // Copyright (c) 2019-2020 Antony Polukhin.
   2 //
   3 // Distributed under the Boost Software License, Version 1.0. (See accompanying
   4 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   5
   6 #ifndef BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
   7 #define BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
   8
   9 #include <boost/pfr/detail/config.hpp>
  10
  11 #include <type_traits>
  12
  13 namespace boost { namespace pfr { namespace detail {
  14
  15 // This function serves as a link-time assert. If linker requires it, then
  16 // `unsafe_declval()` is used at runtime.
  17 void report_if_you_see_link_error_with_this_function() noexcept;
  18
  19 // For returning non default constructible types. Do NOT use at runtime!
  20 //
  21 // GCCs std::declval may not be used in potentionally evaluated contexts,
  22 // so we reinvent it.
  23 template <class T>
  24 constexpr T unsafe_declval() noexcept {
  25     report_if_you_see_link_error_with_this_function();
  26
  27     typename std::remove_reference<T>::type* ptr = 0;
  28     ptr += 42; // suppresses 'null pointer dereference' warnings
  29     return static_cast<T>(*ptr);
  30 }
  31
  32 }}} // namespace boost::pfr::detail
  33
  34
  35 #endif // BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
  36