ceph/src/boost/libs/hana/example/tutorial/introspection.sfinae.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 <string>
   8 #include <vector>
   9 namespace hana = boost::hana;
  10
  11
  12 struct yes { std::string toString() const { return "yes"; } };
  13 struct no { };
  14
  15 //! [optionalToString.sfinae]
  16 template <typename T>
  17 std::string optionalToString(T const& obj) {
  18   auto maybe_toString = hana::sfinae([](auto&& x) -> decltype(x.toString()) {
  19     return x.toString();
  20   });
  21
  22   return maybe_toString(obj).value_or("toString not defined");
  23 }
  24 //! [optionalToString.sfinae]
  25
  26 int main() {
  27 BOOST_HANA_RUNTIME_CHECK(optionalToString(yes{}) == "yes");
  28 BOOST_HANA_RUNTIME_CHECK(optionalToString(no{}) == "toString not defined");
  29
  30 {
  31
  32 //! [maybe_add]
  33 auto maybe_add = hana::sfinae([](auto x, auto y) -> decltype(x + y) {
  34   return x + y;
  35 });
  36
  37 maybe_add(1, 2); // hana::just(3)
  38
  39 std::vector<int> v;
  40 maybe_add(v, "foobar"); // hana::nothing
  41 //! [maybe_add]
  42
  43 }
  44 }