1 // Copyright Louis Dionne 2013-2017 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 main()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 } 60