chain.cpp
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// Copyright Louis Dionne 2013-2022
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/chain.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/optional.hpp>
namespace hana = boost::hana;
BOOST_HANA_CONSTEXPR_LAMBDA auto deref = [](auto x) -> decltype(*x) {
return *x;
};
BOOST_HANA_CONSTEXPR_LAMBDA auto age = [](auto x) -> decltype(x.age) {
return x.age;
};
BOOST_HANA_CONSTEXPR_LAMBDA auto f = [](auto x) {
return hana::chain(hana::sfinae(deref)(x), hana::sfinae(age));
};
struct Person {
unsigned int age;
// ...
};
int main() {
constexpr Person john{30};
// Can't dereference a non-pointer.
BOOST_HANA_CONSTANT_CHECK(f(john) == hana::nothing);
// `int` has no member named `age`.
BOOST_HANA_CONSTANT_CHECK(f(1) == hana::nothing);
// All is good.
BOOST_HANA_CONSTEXPR_CHECK(f(&john) == hana::just(30u));
}