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 toStringyes12struct yes { std::string toString() const { return "yes"; } }; 13 struct no { }; 14 15 //! [optionalToString.sfinae] 16 template <typename T> optionalToString(T const & obj)17std::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 main()26int 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