| 1 | // Copyright Louis Dionne 2013-2022 |
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| 2 | // Distributed under the Boost Software License, Version 1.0. |
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| 3 | // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 4 | |
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| 5 | // Make sure `assert` always triggers an assertion |
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| 6 | #ifdef NDEBUG |
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| 7 | # undef NDEBUG |
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| 8 | #endif |
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| 9 | |
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| 10 | //! [full] |
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| 11 | #include <boost/hana.hpp> |
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| 12 | |
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| 13 | #include <boost/any.hpp> |
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| 14 | #include <cassert> |
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| 15 | #include <string> |
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| 16 | #include <typeindex> |
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| 17 | #include <typeinfo> |
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| 18 | #include <utility> |
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| 19 | namespace hana = boost::hana; |
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| 20 | |
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| 21 | //! [cases] |
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| 22 | template <typename T> |
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| 23 | auto case_ = [](auto f) { |
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| 24 | return hana::make_pair(hana::type_c<T>, f); |
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| 25 | }; |
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| 26 | |
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| 27 | struct default_t; |
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| 28 | auto default_ = case_<default_t>; |
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| 29 | //! [cases] |
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| 30 | |
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| 31 | //! [process] |
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| 32 | template <typename Any, typename Default> |
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| 33 | auto process(Any&, std::type_index const&, Default& default_) { |
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| 34 | return default_(); |
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| 35 | } |
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| 36 | |
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| 37 | template <typename Any, typename Default, typename Case, typename ...Rest> |
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| 38 | auto process(Any& a, std::type_index const& t, Default& default_, |
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| 39 | Case& case_, Rest& ...rest) |
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| 40 | { |
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| 41 | using T = typename decltype(+hana::first(case_))::type; |
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| 42 | return t == typeid(T) ? hana::second(case_)(*boost::unsafe_any_cast<T>(&a)) |
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| 43 | : process(a, t, default_, rest...); |
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| 44 | } |
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| 45 | //! [process] |
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| 46 | |
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| 47 | //! [switch_] |
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| 48 | template <typename Any> |
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| 49 | auto switch_(Any& a) { |
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| 50 | return [&a](auto ...cases_) { |
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| 51 | auto cases = hana::make_tuple(cases_...); |
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| 52 | |
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| 53 | auto default_ = hana::find_if(cases, [](auto const& c) { |
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| 54 | return hana::first(c) == hana::type_c<default_t>; |
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| 55 | }); |
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| 56 | static_assert(default_ != hana::nothing, |
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| 57 | "switch is missing a default_ case" ); |
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| 58 | |
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| 59 | auto rest = hana::filter(cases, [](auto const& c) { |
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| 60 | return hana::first(c) != hana::type_c<default_t>; |
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| 61 | }); |
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| 62 | |
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| 63 | return hana::unpack(rest, [&](auto& ...rest) { |
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| 64 | return process(a, a.type(), hana::second(*default_), rest...); |
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| 65 | }); |
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| 66 | }; |
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| 67 | } |
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| 68 | //! [switch_] |
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| 69 | //! [full] |
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| 70 | |
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| 71 | |
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| 72 | int main() { |
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| 73 | using namespace std::literals; |
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| 74 | |
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| 75 | { |
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| 76 | |
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| 77 | //! [usage] |
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| 78 | boost::any a = 'x'; |
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| 79 | std::string r = switch_(a)( |
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| 80 | case_<int>([](auto i) { return "int: "s + std::to_string(i); }), |
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| 81 | case_<char>([](auto c) { return "char: "s + std::string{c}; }), |
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| 82 | default_([] { return "unknown"s ; }) |
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| 83 | ); |
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| 84 | |
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| 85 | assert(r == "char: x"s ); |
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| 86 | //! [usage] |
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| 87 | |
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| 88 | }{ |
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| 89 | |
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| 90 | //! [result_inference] |
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| 91 | boost::any a = 'x'; |
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| 92 | auto r = switch_(a)( |
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| 93 | case_<int>([](auto) -> int { return 1; }), |
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| 94 | case_<char>([](auto) -> long { return 2l; }), |
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| 95 | default_([]() -> long long { return 3ll; }) |
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| 96 | ); |
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| 97 | |
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| 98 | // r is inferred to be a long long |
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| 99 | static_assert(std::is_same<decltype(r), long long>{}, "" ); |
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| 100 | assert(r == 2ll); |
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| 101 | //! [result_inference] |
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| 102 | |
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| 103 | } |
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| 104 | |
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| 105 | } |
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| 106 | |
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