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 
__anone29c39820102(auto x) 21 BOOST_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()30 int 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