| 1 | // Copyright Louis Dionne 2013-2022 |
|---|---|
| 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 | } |
| 45 |