1// Copyright Louis Dionne 2013-2022
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#include <boost/hana/assert.hpp>
6#include <boost/hana/first.hpp>
7#include <boost/hana/fold_left.hpp>
8#include <boost/hana/insert.hpp>
9#include <boost/hana/integral_constant.hpp>
10#include <boost/hana/map.hpp>
11#include <boost/hana/pair.hpp>
12#include <boost/hana/second.hpp>
13#include <boost/hana/type.hpp>
14namespace hana = boost::hana;
15
16
17int main() {
18 // Given a map of (key, value) pairs, returns a map of (value, key) pairs.
19 // This requires both the keys and the values to be compile-time Comparable.
20 auto invert = [](auto map) {
21 return hana::fold_left(map, hana::make_map(), [](auto map, auto pair) {
22 return hana::insert(map, hana::make_pair(hana::second(pair), hana::first(pair)));
23 });
24 };
25
26 auto m = hana::make_map(
27 hana::make_pair(hana::type_c<int>, hana::int_c<1>),
28 hana::make_pair(hana::type_c<float>, hana::int_c<2>),
29 hana::make_pair(hana::int_c<3>, hana::type_c<void>)
30 );
31
32 BOOST_HANA_CONSTANT_CHECK(invert(m) ==
33 hana::make_map(
34 hana::make_pair(hana::int_c<1>, hana::type_c<int>),
35 hana::make_pair(hana::int_c<2>, hana::type_c<float>),
36 hana::make_pair(hana::type_c<void>, hana::int_c<3>)
37 )
38 );
39}
40

source code of boost/libs/hana/example/map/foldable.cpp