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)
5 #include <boost/hana.hpp>
9 namespace hana = boost::hana;
12 struct yes { std::string toString() const { return "yes"; } };
15 //! [optionalToString.sfinae]
17 std::string optionalToString(T const& obj) {
18 auto maybe_toString = hana::sfinae([](auto&& x) -> decltype(x.toString()) {
22 return maybe_toString(obj).value_or("toString not defined");
24 //! [optionalToString.sfinae]
27 BOOST_HANA_RUNTIME_CHECK(optionalToString(yes{}) == "yes");
28 BOOST_HANA_RUNTIME_CHECK(optionalToString(no{}) == "toString not defined");
33 auto maybe_add = hana::sfinae([](auto x, auto y) -> decltype(x + y) {
37 maybe_add(1, 2); // hana::just(3)
40 maybe_add(v, "foobar"); // hana::nothing