60 lines
2.4 KiB
C++
60 lines
2.4 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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// Make sure assert always triggers an assertion
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#ifdef NDEBUG
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# undef NDEBUG
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#endif
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//////////////////////////////////////////////////////////////////////////////
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// Important: Keep this file in sync with the Overview in the README
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//////////////////////////////////////////////////////////////////////////////
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#include <boost/hana.hpp>
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#include <cassert>
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#include <string>
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namespace hana = boost::hana;
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using namespace hana::literals;
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struct Fish { std::string name; };
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struct Cat { std::string name; };
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struct Dog { std::string name; };
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int main() {
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// Sequences capable of holding heterogeneous objects, and algorithms
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// to manipulate them.
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auto animals = hana::make_tuple(Fish{"Nemo"}, Cat{"Garfield"}, Dog{"Snoopy"});
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auto names = hana::transform(animals, [](auto a) {
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return a.name;
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});
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assert(hana::reverse(names) == hana::make_tuple("Snoopy", "Garfield", "Nemo"));
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// No compile-time information is lost: even if `animals` can't be a
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// constant expression because it contains strings, its length is constexpr.
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static_assert(hana::length(animals) == 3u, "");
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// Computations on types can be performed with the same syntax as that of
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// normal C++. Believe it or not, everything is done at compile-time.
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auto animal_types = hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Cat&>, hana::type_c<Dog*>);
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auto animal_ptrs = hana::filter(animal_types, [](auto a) {
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return hana::traits::is_pointer(a);
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});
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static_assert(animal_ptrs == hana::make_tuple(hana::type_c<Fish*>, hana::type_c<Dog*>), "");
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// And many other goodies to make your life easier, including:
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// 1. Access to elements in a tuple with a sane syntax.
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static_assert(animal_ptrs[0_c] == hana::type_c<Fish*>, "");
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static_assert(animal_ptrs[1_c] == hana::type_c<Dog*>, "");
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// 2. Unroll loops at compile-time without hassle.
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std::string s;
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hana::int_c<10>.times([&]{ s += "x"; });
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// equivalent to s += "x"; s += "x"; ... s += "x";
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// 3. Easily check whether an expression is valid.
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// This is usually achieved with complex SFINAE-based tricks.
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auto has_name = hana::is_valid([](auto&& x) -> decltype((void)x.name) { });
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static_assert(has_name(animals[0_c]), "");
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static_assert(!has_name(1), "");
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}
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