ceph/src/boost/libs/hana/example/wandbox.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.hpp>
   6
   7 #include <functional>
   8 #include <iostream>
   9 #include <string>
  10 #include <type_traits>
  11 #include <utility>
  12 namespace hana = boost::hana;
  13 using namespace hana::literals;
  14 using namespace std::literals;
  15
  16
  17 //////////////////////////////////////////////////////////////////////////////
  18 // Welcome to Hana!
  19 //
  20 // You can play around and press 'Run' at the bottom of this file to compile
  21 // and run this code.
  22 //
  23 // To get you started, here's a small JSON generator written with Hana
  24 // (this is explained in the tutorial if you're interested):
  25 //////////////////////////////////////////////////////////////////////////////
  26
  27 // 1. Define some utilities
  28 template <typename Xs>
  29 std::string join(Xs&& xs, std::string sep) {
  30   return hana::fold(hana::intersperse(std::forward<Xs>(xs), sep), "", std::plus<>{});
  31 }
  32
  33 std::string quote(std::string s) { return "\"" + s + "\""; }
  34
  35 template <typename T>
  36 auto to_json(T const& x) -> decltype(std::to_string(x)) {
  37   return std::to_string(x);
  38 }
  39
  40 std::string to_json(char c) { return quote({c}); }
  41 std::string to_json(std::string s) { return quote(s); }
  42
  43
  44 // 2. Define how to print user-defined types
  45 template <typename T>
  46   std::enable_if_t<hana::Struct<T>::value,
  47 std::string> to_json(T const& x) {
  48   auto json = hana::transform(hana::keys(x), [&](auto name) {
  49     auto const& member = hana::at_key(x, name);
  50     return quote(hana::to<char const*>(name)) + " : " + to_json(member);
  51   });
  52
  53   return "{" + join(std::move(json), ", ") + "}";
  54 }
  55
  56 // 3. Define how to print Sequences
  57 template <typename Xs>
  58   std::enable_if_t<hana::Sequence<Xs>::value,
  59 std::string> to_json(Xs const& xs) {
  60   auto json = hana::transform(xs, [](auto const& x) {
  61     return to_json(x);
  62   });
  63
  64   return "[" + join(std::move(json), ", ") + "]";
  65 }
  66
  67
  68 // 4. Create your own types and make them compatible with Hana.
  69 //    This can be done intrusively or non-intrusively.
  70 struct Car {
  71   BOOST_HANA_DEFINE_STRUCT(Car,
  72     (std::string, brand),
  73     (std::string, model)
  74   );
  75 };
  76
  77 int main() {
  78   // 5. Generate beautiful JSON without hassle. Enjoy!
  79   auto cars = hana::make_tuple(
  80     Car{"BMW", "Z3"},
  81     Car{"Audi", "A4"},
  82     Car{"Ferrari", "F40"},
  83     Car{"Lamborghini", "Diablo"}
  84     // ...
  85   );
  86
  87   std::cout << to_json(cars) << std::endl;
  88 }