ceph/src/boost/libs/hana/test/map/at_key.stackoverflow.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/at_key.hpp>
   7 #include <boost/hana/fold_left.hpp>
   8 #include <boost/hana/integral_constant.hpp>
   9 #include <boost/hana/map.hpp>
  10 #include <boost/hana/pair.hpp>
  11 #include <boost/hana/range.hpp>
  12 namespace hana = boost::hana;
  13
  14
  15 //
  16 // Make sure that http://stackoverflow.com/q/32702383/627587 works.
  17 //
  18
  19 auto at = [](auto& map, auto key) -> auto& {
  20     return map[key];
  21 };
  22
  23 template <typename Map, typename Keys>
  24 auto& traverse(Map& map, Keys const& keys){
  25     return hana::fold_left(keys, map, at);
  26 }
  27
  28 int main() {
  29     auto xs = hana::make_map(hana::make_pair(hana::int_c<0>,
  30                 hana::make_map(hana::make_pair(hana::int_c<1>,
  31                     hana::make_map(hana::make_pair(hana::int_c<2>,
  32                         hana::make_map(hana::make_pair(hana::int_c<3>, 10))))))));
  33
  34     int& i = traverse(xs, hana::range_c<int, 0, 4>);
  35     BOOST_HANA_RUNTIME_CHECK(i == 10);
  36     i = 99;
  37     BOOST_HANA_RUNTIME_CHECK(traverse(xs, hana::range_c<int, 0, 4>) == 99);
  38 }