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/assert.hpp> 6 #include <boost/hana/chain.hpp> 7 #include <boost/hana/config.hpp> 8 #include <boost/hana/equal.hpp> 9 #include <boost/hana/optional.hpp> 10 namespace hana = boost::hana; 11 12 13 BOOST_HANA_CONSTEXPR_LAMBDA auto deref = [](auto x) -> decltype(*x) { 14 return *x; 15 }; 16 17 BOOST_HANA_CONSTEXPR_LAMBDA auto age = [](auto x) -> decltype(x.age) { 18 return x.age; 19 }; 20 __anone385aea90102(auto x) 21BOOST_HANA_CONSTEXPR_LAMBDA auto f = [](auto x) { 22 return hana::chain(hana::sfinae(deref)(x), hana::sfinae(age)); 23 }; 24 25 struct Person { 26 unsigned int age; 27 // ... 28 }; 29 main()30int main() { 31 constexpr Person john{30}; 32 33 // Can't dereference a non-pointer. 34 BOOST_HANA_CONSTANT_CHECK(f(john) == hana::nothing); 35 36 // `int` has no member named `age`. 37 BOOST_HANA_CONSTANT_CHECK(f(1) == hana::nothing); 38 39 // All is good. 40 BOOST_HANA_CONSTEXPR_CHECK(f(&john) == hana::just(30u)); 41 } 42