Create a printf-style format string at compile-time

compile_time_printf.cpp

#include <type_traits>
#include <boost/hana.hpp>
#include <boost/hana/ext/std/type_traits.hpp>
#include <iostream>
using namespace hana;


constexpr auto formats = hana::make_map(
    hana::make<hana::Pair>(hana::type<int>, hana::string<'%', 'd'>),
    hana::make<hana::Pair>(hana::type<float>, hana::string<'%', 'f'>),
    hana::make<hana::Pair>(hana::type<char const*>, hana::string<'%', 's'>)
);

struct concat_strings {
    template <char ...s1, char ...s2>
    constexpr auto operator()(hana::_string<s1...>, hana::_string<s2...>) const
    { return hana::string<s1..., s2...>; }
};

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 `hana::demux(...)(...)`:
    //
    // [](auto token) {
    //     return hana::from_just(hana::lookup(formats,
    //                                         hana::decltype_(token)));
    // }
    auto format_string_tokens = hana::adjust(tokens,
        hana::compose(hana::not_, hana::is_a<hana::String>),

        hana::demux(hana::from_just)(
            hana::compose(hana::partial(hana::lookup, formats),
                          hana::decltype_)
        )
    );

    auto format_string = hana::foldl(
        format_string_tokens, hana::string<>, concat_strings{}
    );

    auto variables = hana::filter(tokens,
        hana::compose(hana::not_, hana::is_a<hana::String>)
    );

    return hana::prepend(format_string, variables);
}


template <char ...s>
char const string_literal[] = {s..., '\0'};

template <char ...s>
auto to_string_literal(hana::_string<s...> const&)
{ return string_literal<s...>; }

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) {
        return std::printf(to_string_literal(fmt), args...);
    });
}