ceph/src/boost/libs/hana/example/misc/printf.cpp

   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 #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>
  23 namespace hana = boost::hana;
  24
  25
  26 constexpr 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
  32 template <typename ...Tokens>
  33 constexpr 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
  52 int 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="), a
  59         , BOOST_HANA_STRING(" second="), b
  60         , BOOST_HANA_STRING(" third="), c
  61     );
  62
  63     hana::unpack(args, [](auto fmt, auto ...args) {
  64         std::printf(hana::to<char const*>(fmt), args...);
  65     });
  66 }