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1 | // Copyright Louis Dionne 2013-2016 |
2 | // Distributed under the Boost Software License, Version 1.0. | |
3 | // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) | |
4 | ||
5 | // Make sure assert always triggers an assertion | |
6 | #ifdef NDEBUG | |
7 | # undef NDEBUG | |
8 | #endif | |
9 | ||
10 | ////////////////////////////////////////////////////////////////////////////// | |
11 | // Important: Keep this file in sync with the Overview in the README | |
12 | ////////////////////////////////////////////////////////////////////////////// | |
13 | #include <boost/hana.hpp> | |
14 | #include <cassert> | |
15 | #include <string> | |
16 | namespace hana = boost::hana; | |
17 | using namespace hana::literals; | |
18 | ||
19 | struct Fish { std::string name; }; | |
20 | struct Cat { std::string name; }; | |
21 | struct Dog { std::string name; }; | |
22 | ||
23 | int main() { | |
24 | // Sequences capable of holding heterogeneous objects, and algorithms | |
25 | // to manipulate them. | |
26 | auto animals = hana::make_tuple(Fish{"Nemo"}, Cat{"Garfield"}, Dog{"Snoopy"}); | |
27 | auto names = hana::transform(animals, [](auto a) { | |
28 | return a.name; | |
29 | }); | |
30 | assert(hana::reverse(names) == hana::make_tuple("Snoopy", "Garfield", "Nemo")); | |
31 | ||
32 | // No compile-time information is lost: even if `animals` can't be a | |
33 | // constant expression because it contains strings, its length is constexpr. | |
34 | static_assert(hana::length(animals) == 3u, ""); | |
35 | ||
36 | // Computations on types can be performed with the same syntax as that of | |
37 | // normal C++. Believe it or not, everything is done at compile-time. | |
38 | auto animal_types = hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Cat&>, hana::type_c<Dog*>); | |
39 | auto animal_ptrs = hana::filter(animal_types, [](auto a) { | |
40 | return hana::traits::is_pointer(a); | |
41 | }); | |
42 | static_assert(animal_ptrs == hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Dog*>), ""); | |
43 | ||
44 | // And many other goodies to make your life easier, including: | |
45 | // 1. Access to elements in a tuple with a sane syntax. | |
46 | static_assert(animal_ptrs[0_c] == hana::type_c<Fish*>, ""); | |
47 | static_assert(animal_ptrs[1_c] == hana::type_c<Dog*>, ""); | |
48 | ||
49 | // 2. Unroll loops at compile-time without hassle. | |
50 | std::string s; | |
51 | hana::int_c<10>.times([&]{ s += "x"; }); | |
52 | // equivalent to s += "x"; s += "x"; ... s += "x"; | |
53 | ||
54 | // 3. Easily check whether an expression is valid. | |
55 | // This is usually achieved with complex SFINAE-based tricks. | |
56 | auto has_name = hana::is_valid([](auto&& x) -> decltype((void)x.name) { }); | |
57 | static_assert(has_name(animals[0_c]), ""); | |
58 | static_assert(!has_name(1), ""); | |
59 | } |