EQ2EMu/EQ2/source/depends/boost_1_72_0/libs/hana/example/chain.cpp

// Copyright Louis Dionne 2013-2017
// 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));
}