ceph/src/boost/libs/hana/example/chain.cpp

   1 // Copyright Louis Dionne 2013-2017
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/assert.hpp>
   6 #include <boost/hana/chain.hpp>
   7 #include <boost/hana/config.hpp>
   8 #include <boost/hana/equal.hpp>
   9 #include <boost/hana/optional.hpp>
  10 namespace hana = boost::hana;
  11
  12
  13 BOOST_HANA_CONSTEXPR_LAMBDA auto deref = [](auto x) -> decltype(*x) {
  14     return *x;
  15 };
  16
  17 BOOST_HANA_CONSTEXPR_LAMBDA auto age = [](auto x) -> decltype(x.age) {
  18     return x.age;
  19 };
  20
  21 BOOST_HANA_CONSTEXPR_LAMBDA auto f = [](auto x) {
  22     return hana::chain(hana::sfinae(deref)(x), hana::sfinae(age));
  23 };
  24
  25 struct Person {
  26     unsigned int age;
  27     // ...
  28 };
  29
  30 int main() {
  31     constexpr Person john{30};
  32
  33     // Can't dereference a non-pointer.
  34     BOOST_HANA_CONSTANT_CHECK(f(john) == hana::nothing);
  35
  36     // `int` has no member named `age`.
  37     BOOST_HANA_CONSTANT_CHECK(f(1) == hana::nothing);
  38
  39     // All is good.
  40     BOOST_HANA_CONSTEXPR_CHECK(f(&john) == hana::just(30u));
  41 }