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

   1 // Copyright Louis Dionne 2013-2016
   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 // Make sure assert always triggers an assertion
   6 #ifdef NDEBUG
   7 #   undef NDEBUG
   8 #endif
   9
  10 //////////////////////////////////////////////////////////////////////////////
  11 // Important: Keep this file in sync with the Overview in the README
  12 //////////////////////////////////////////////////////////////////////////////
  13 #include <boost/hana.hpp>
  14 #include <cassert>
  15 #include <string>
  16 namespace hana = boost::hana;
  17 using namespace hana::literals;
  18
  19 struct Fish { std::string name; };
  20 struct Cat  { std::string name; };
  21 struct Dog  { std::string name; };
  22
  23 int main() {
  24   // Sequences capable of holding heterogeneous objects, and algorithms
  25   // to manipulate them.
  26   auto animals = hana::make_tuple(Fish{"Nemo"}, Cat{"Garfield"}, Dog{"Snoopy"});
  27   auto names = hana::transform(animals, [](auto a) {
  28     return a.name;
  29   });
  30   assert(hana::reverse(names) == hana::make_tuple("Snoopy", "Garfield", "Nemo"));
  31
  32   // No compile-time information is lost: even if `animals` can't be a
  33   // constant expression because it contains strings, its length is constexpr.
  34   static_assert(hana::length(animals) == 3u, "");
  35
  36   // Computations on types can be performed with the same syntax as that of
  37   // normal C++. Believe it or not, everything is done at compile-time.
  38   auto animal_types = hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Cat&>, hana::type_c<Dog*>);
  39   auto animal_ptrs = hana::filter(animal_types, [](auto a) {
  40     return hana::traits::is_pointer(a);
  41   });
  42   static_assert(animal_ptrs == hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Dog*>), "");
  43
  44   // And many other goodies to make your life easier, including:
  45   // 1. Access to elements in a tuple with a sane syntax.
  46   static_assert(animal_ptrs[0_c] == hana::type_c<Fish*>, "");
  47   static_assert(animal_ptrs[1_c] == hana::type_c<Dog*>, "");
  48
  49   // 2. Unroll loops at compile-time without hassle.
  50   std::string s;
  51   hana::int_c<10>.times([&]{ s += "x"; });
  52   // equivalent to s += "x"; s += "x"; ... s += "x";
  53
  54   // 3. Easily check whether an expression is valid.
  55   //    This is usually achieved with complex SFINAE-based tricks.
  56   auto has_name = hana::is_valid([](auto&& x) -> decltype((void)x.name) { });
  57   static_assert(has_name(animals[0_c]), "");
  58   static_assert(!has_name(1), "");
  59 }