cpp_constexpr_string_interpolation

ct_string.hpp

#include <boost/mp11/map.hpp>
#include <boost/mp11/list.hpp>

namespace ct {

    using namespace boost::mp11;

    template <class C, C... Cs>
    struct nttp_string {
        constexpr static C data[sizeof...(Cs)] = {Cs...};
        constexpr static auto value = []{ return data; };
        constexpr static char const* c_str() noexcept {
            return data;
        }
    };

    template <class S1, class S2>
    struct nttp_string_concat;

    template <class C, C... S1Cs, C... S2Cs>
    struct nttp_string_concat<nttp_string<C, S1Cs...>, nttp_string<C, S2Cs...>> {
        using type = nttp_string<C, S1Cs..., S2Cs...>;
    };

    enum class state {
        waiting_for_opener,
        waiting_for_closer
    };

    template <class Format, class Table, class Result, class Key, state State = state::waiting_for_opener>
    struct nttp_string_formatter_impl;

    template <class C, class Table, C... ResultCs, class Key>
    struct nttp_string_formatter_impl<nttp_string<C>, Table, nttp_string<C, ResultCs...>, Key, state::waiting_for_opener> {
        constexpr static auto value = nttp_string<C, ResultCs...>();
    };

    template <class C, C Front, C... FormatCs, class Table, C... ResultCs, class Key>
    struct nttp_string_formatter_impl<nttp_string<C, Front, FormatCs...>, Table, nttp_string<C, ResultCs...>, Key, state::waiting_for_opener> {
        constexpr static auto value = nttp_string_formatter_impl<
            nttp_string<C, FormatCs...>, Table, nttp_string<C, ResultCs..., Front>, Key, state::waiting_for_opener
        >::value;
    };

    template <class C, C... FormatCs, class Table, C... ResultCs, class Key>
    struct nttp_string_formatter_impl<nttp_string<C, C('{'), FormatCs...>, Table, nttp_string<C, ResultCs...>, Key, state::waiting_for_opener> {
        constexpr static auto value = nttp_string_formatter_impl<
            nttp_string<C, FormatCs...>, Table, nttp_string<C, ResultCs...>, nttp_string<C>, state::waiting_for_closer
        >::value;
    };

    template <class C, C Front, C... FormatCs, class Table, class Result, C... KeyCs>
    struct nttp_string_formatter_impl<nttp_string<C, Front, FormatCs...>, Table, Result, nttp_string<C, KeyCs...>, state::waiting_for_closer> {
        constexpr static auto value = nttp_string_formatter_impl<
            nttp_string<C, FormatCs...>, Table, Result, nttp_string<C, KeyCs..., Front>, state::waiting_for_closer
        >::value;
    };

    template <class C, C... FormatCs, class Table, class Result, C... KeyCs>
    struct nttp_string_formatter_impl<nttp_string<C, C('}'), FormatCs...>, Table, Result, nttp_string<C, KeyCs...>, state::waiting_for_closer> {
        using substitution = mp_second<mp_map_find<Table, nttp_string<C, KeyCs...>>>;
        constexpr static auto value = nttp_string_formatter_impl<
            nttp_string<C, FormatCs...>, Table, typename nttp_string_concat<Result, substitution>::type, nttp_string<C, KeyCs...>, state::waiting_for_opener
        >::value;
    };

    template <class Format, class Table>
    struct nttp_string_formatter;

    template <class C, C... Cs, class Table>
    struct nttp_string_formatter<nttp_string<C, Cs...>, Table> {
        constexpr static auto value = nttp_string_formatter_impl<nttp_string<C, Cs...>, Table, nttp_string<C>, nttp_string<C>>::value;
    };

    template <class Format, class Table = mp_list<>>
    struct formatted_nttp_string_builder {
        template <class Key, class Value>
        constexpr auto operator()(Key, Value) {
            return formatted_nttp_string_builder<Format, mp_map_insert<Table, mp_list<Key, Value>>>();
        }

        constexpr auto operator()() {
            return nttp_string_formatter<Format, Table>::value;
        }
    };

    namespace string_literals {
        template <class C, C... Cs>
        constexpr auto operator""_s() noexcept {
            return nttp_string<C, Cs...>();
        }

        template <class C, C... Cs>
        constexpr auto operator""_f() noexcept {
            return formatted_nttp_string_builder<nttp_string<C, Cs...>>();
        }
    }
}

using namespace ct::string_literals;

constexpr auto s = "Hello {name}, today is {date}!"_f("name"_s, "Foo"_s)("date"_s, "Friday"_s)();

char const* cs = s.c_str();