42 lines
1.0 KiB
C++
42 lines
1.0 KiB
C++
// Copyright Louis Dionne 2013-2017
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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#include <boost/hana/assert.hpp>
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#include <boost/hana/chain.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/equal.hpp>
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#include <boost/hana/optional.hpp>
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namespace hana = boost::hana;
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BOOST_HANA_CONSTEXPR_LAMBDA auto deref = [](auto x) -> decltype(*x) {
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return *x;
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};
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BOOST_HANA_CONSTEXPR_LAMBDA auto age = [](auto x) -> decltype(x.age) {
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return x.age;
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};
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BOOST_HANA_CONSTEXPR_LAMBDA auto f = [](auto x) {
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return hana::chain(hana::sfinae(deref)(x), hana::sfinae(age));
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};
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struct Person {
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unsigned int age;
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// ...
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};
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int main() {
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constexpr Person john{30};
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// Can't dereference a non-pointer.
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BOOST_HANA_CONSTANT_CHECK(f(john) == hana::nothing);
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// `int` has no member named `age`.
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BOOST_HANA_CONSTANT_CHECK(f(1) == hana::nothing);
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// All is good.
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BOOST_HANA_CONSTEXPR_CHECK(f(&john) == hana::just(30u));
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}
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