| 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 | ////////////////////////////////////////////////////////////////////////////// |
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| 11 | // Important: Keep this file in sync with the Overview in the README |
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| 12 | ////////////////////////////////////////////////////////////////////////////// |
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| 13 | #include <boost/hana.hpp> |
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| 14 | #include <cassert> |
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| 15 | #include <string> |
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| 16 | namespace hana = boost::hana; |
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| 17 | using namespace hana::literals; |
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| 18 | |
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| 19 | struct Fish { std::string name; }; |
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| 20 | struct Cat { std::string name; }; |
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| 21 | struct Dog { std::string name; }; |
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| 22 | |
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| 23 | int main() { |
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| 24 | // Sequences capable of holding heterogeneous objects, and algorithms |
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| 25 | // to manipulate them. |
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| 26 | auto animals = hana::make_tuple(Fish{.name: "Nemo" }, Cat{.name: "Garfield" }, Dog{.name: "Snoopy" }); |
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| 27 | auto names = hana::transform(animals, [](auto a) { |
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| 28 | return a.name; |
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| 29 | }); |
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| 30 | assert(hana::reverse(names) == hana::make_tuple("Snoopy" , "Garfield" , "Nemo" )); |
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| 31 | |
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| 32 | // No compile-time information is lost: even if `animals` can't be a |
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| 33 | // constant expression because it contains strings, its length is constexpr. |
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| 34 | static_assert(hana::length(animals) == 3u, "" ); |
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| 35 | |
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| 36 | // Computations on types can be performed with the same syntax as that of |
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| 37 | // normal C++. Believe it or not, everything is done at compile-time. |
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| 38 | auto animal_types = hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Cat&>, hana::type_c<Dog*>); |
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| 39 | auto animal_ptrs = hana::filter(animal_types, [](auto a) { |
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| 40 | return hana::traits::is_pointer(a); |
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| 41 | }); |
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| 42 | static_assert(animal_ptrs == hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Dog*>), "" ); |
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| 43 | |
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| 44 | // And many other goodies to make your life easier, including: |
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| 45 | // 1. Access to elements in a tuple with a sane syntax. |
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| 46 | static_assert(animal_ptrs[0_c] == hana::type_c<Fish*>, "" ); |
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| 47 | static_assert(animal_ptrs[1_c] == hana::type_c<Dog*>, "" ); |
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| 48 | |
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| 49 | // 2. Unroll loops at compile-time without hassle. |
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| 50 | std::string s; |
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| 51 | hana::int_c<10>.times([&]{ s += "x" ; }); |
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| 52 | // equivalent to s += "x"; s += "x"; ... s += "x"; |
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| 53 | |
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| 54 | // 3. Easily check whether an expression is valid. |
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| 55 | // This is usually achieved with complex SFINAE-based tricks. |
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| 56 | auto has_name = hana::is_valid([](auto&& x) -> decltype((void)x.name) { }); |
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| 57 | static_assert(has_name(animals[0_c]), "" ); |
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| 58 | static_assert(!has_name(1), "" ); |
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| 59 | } |
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| 60 | |
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