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/adjust_if.hpp>
6#include <boost/hana/at_key.hpp>
7#include <boost/hana/core/is_a.hpp>
8#include <boost/hana/core/to.hpp>
9#include <boost/hana/filter.hpp>
10#include <boost/hana/functional/compose.hpp>
11#include <boost/hana/functional/partial.hpp>
12#include <boost/hana/map.hpp>
13#include <boost/hana/not.hpp>
14#include <boost/hana/pair.hpp>
15#include <boost/hana/prepend.hpp>
16#include <boost/hana/string.hpp>
17#include <boost/hana/sum.hpp>
18#include <boost/hana/tuple.hpp>
19#include <boost/hana/type.hpp>
20#include <boost/hana/unpack.hpp>
21
22#include <cstdio>
23namespace hana = boost::hana;
24
25
26constexpr auto formats = hana::make_map(
27 hana::make_pair(hana::type_c<int>, hana::string_c<'%', 'd'>),
28 hana::make_pair(hana::type_c<float>, hana::string_c<'%', 'f'>),
29 hana::make_pair(hana::type_c<char const*>, hana::string_c<'%', 's'>)
30);
31
32template <typename ...Tokens>
33constexpr auto format(Tokens ...tokens_) {
34 auto tokens = hana::make_tuple(tokens_...);
35
36 // If you don't care about constexpr-ness of `format`, you can use
37 // this lambda instead of `compose(partial(...), typeid_)`:
38 //
39 // [](auto token) {
40 // return formats[typeid_(token)];
41 // }
42 auto format_string_tokens = hana::adjust_if(tokens,
43 hana::compose(hana::not_, hana::is_a<hana::string_tag>),
44 hana::compose(hana::partial(hana::at_key, formats), hana::typeid_)
45 );
46
47 auto format_string = hana::sum<hana::string_tag>(format_string_tokens);
48 auto variables = hana::filter(tokens, hana::compose(hana::not_, hana::is_a<hana::string_tag>));
49 return hana::prepend(variables, format_string);
50}
51
52int main() {
53 int a = 1;
54 float b = 1.3;
55 char const* c = "abcdef";
56
57 auto args = format(
58 BOOST_HANA_STRING("first="), tokens_: a
59 , BOOST_HANA_STRING(" second="), tokens_: b
60 , BOOST_HANA_STRING(" third="), tokens_: c
61 );
62
63 hana::unpack(args, [](auto fmt, auto ...args) {
64 std::printf(format: hana::to<char const*>(fmt), args...);
65 });
66}
67

source code of boost/libs/hana/example/misc/printf.cpp