// Copyright Louis Dionne 2013-2017 // Distributed under the Boost Software License, Version 1.0. // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) #include <boost/hana/adjust_if.hpp> #include <boost/hana/at_key.hpp> #include <boost/hana/core/is_a.hpp> #include <boost/hana/core/to.hpp> #include <boost/hana/filter.hpp> #include <boost/hana/functional/compose.hpp> #include <boost/hana/functional/partial.hpp> #include <boost/hana/map.hpp> #include <boost/hana/not.hpp> #include <boost/hana/pair.hpp> #include <boost/hana/prepend.hpp> #include <boost/hana/string.hpp> #include <boost/hana/sum.hpp> #include <boost/hana/tuple.hpp> #include <boost/hana/type.hpp> #include <boost/hana/unpack.hpp> #include <cstdio> namespace hana = boost::hana; constexpr auto formats = hana::make_map( hana::make_pair(hana::type_c<int>, hana::string_c<'%', 'd'>), hana::make_pair(hana::type_c<float>, hana::string_c<'%', 'f'>), hana::make_pair(hana::type_c<char const*>, hana::string_c<'%', 's'>) ); template <typename ...Tokens> constexpr auto format(Tokens ...tokens_) { auto tokens = hana::make_tuple(tokens_...); // If you don't care about constexpr-ness of `format`, you can use // this lambda instead of `compose(partial(...), typeid_)`: // // [](auto token) { // return formats[typeid_(token)]; // } auto format_string_tokens = hana::adjust_if(tokens, hana::compose(hana::not_, hana::is_a<hana::string_tag>), hana::compose(hana::partial(hana::at_key, formats), hana::typeid_) ); auto format_string = hana::sum<hana::string_tag>(format_string_tokens); auto variables = hana::filter(tokens, hana::compose(hana::not_, hana::is_a<hana::string_tag>)); return hana::prepend(variables, format_string); } int main() { int a = 1; float b = 1.3; char const* c = "abcdef"; auto args = format( BOOST_HANA_STRING("first="), a , BOOST_HANA_STRING(" second="), b , BOOST_HANA_STRING(" third="), c ); hana::unpack(args, [](auto fmt, auto ...args) { std::printf(hana::to<char const*>(fmt), args...); }); }