I'm trying to generate a simple format string for fmt at compile time, but I can't quite figure out the string concatenation. I'm limited to c 14.

What I'd like is to be able to generate a format string for N items, so it could be used as follows: auto my_string = fmt::format(format_string_struct<2>::format_string, "Item", "key1", "value1", "key2", "value2");

The generated format string would be: "{} {}={} {}={}", resulting in a formatted string of "Item key1=value1 key2=value2". The base of the format string would be "{}", and it appends a " {}={}" for each N.

I'm trying to use recursive templates, but I just can't quite get everything to work. I got some code from Concatenate compile-time strings in a template at compile time? for concatenating strings (though I get a undefined reference to `concat_impl<&base_format_string_struct::format_string, std::integer_sequence<int, 0, 1>, &format_string_parameter_struct::parameter_string, std::integer_sequence<int, 0, 1, 2, 3, 4, 5, 6, 7> >::value' error)

template<int...I> using is      = std::integer_sequence<int,I...>;
template<int N>   using make_is = std::make_integer_sequence<int,N>;

constexpr auto size(const char*s) { int i = 0; while(*s!=0){  i;  s;} return i; }

template<const char*, typename, const char*, typename>
struct concat_impl;

template<const char* S1, int... I1, const char* S2, int... I2>
struct concat_impl<S1, is<I1...>, S2, is<I2...>> {
    static constexpr const char value[]
    {
        S1[I1]..., S2[I2]..., 0
    };
};

template<const char* S1, const char* S2>
constexpr auto concat {
    concat_impl<S1, make_is<size(S1)>, S2, make_is<size(S2)>>::value
};

struct base_format_string_struct {
    static constexpr const char format_string[] = "{}";    
};

struct format_string_parameter_struct {
    static constexpr const char parameter_string[] = " {}=\"{}\"";
};

template <int arg_count, const char[]... params>
struct format_string_struct:
    public format_string_struct<arg_count - 1, format_string_parameter_struct, params...> {
};

template <int arg_count>
struct format_string_struct:
    public format_string_struct<arg_count - 1, format_string_parameter_struct> {
};

template <const char[]... params>
struct format_string_struct<0> {
  static const char* format_string() {
    return concat<base_format_string_struct, params...>;
  }
};

CodePudding user response：

You have several typos (and a concat limited to 2 argument).

template<int...I> using is      = std::integer_sequence<int,I...>;
template<int N>   using make_is = std::make_integer_sequence<int,N>;

constexpr auto size(const char*s) { int i = 0; while(*s!=0){  i;  s;} return i; }

template<const char*, typename, const char*, typename>
struct concat_impl;

template<const char* S1, int... I1, const char* S2, int... I2>
struct concat_impl<S1, is<I1...>, S2, is<I2...>> {
    static constexpr const char value[]
    {
        S1[I1]..., S2[I2]..., 0
    };
};

template<const char* S1, const char* S2, const char*... Ss>
struct concat
{
    constexpr static const char* value = concat_impl<S1, make_is<size(S1)>, concat<S2, Ss...>::value, make_is<size(concat<S2, Ss...>::value)>>::value;
};

template<const char* S1, const char* S2>
struct concat<S1, S2>
{
    constexpr static const char* value = concat_impl<S1, make_is<size(S1)>, S2, make_is<size(S2)>>::value;
};

struct base_format_string_struct {
    static constexpr const char format_string[] = "{}";    
};

struct format_string_parameter_struct {
    static constexpr const char parameter_string[] = " {}=\"{}\"";
};

template <int arg_count, const char*... params>
struct format_string_struct:
    public format_string_struct<arg_count - 1, format_string_parameter_struct::parameter_string, params...> {
};

template <int arg_count>
struct format_string_struct<arg_count>:
    public format_string_struct<arg_count - 1, format_string_parameter_struct::parameter_string> {
};

template <const char*... params>
struct format_string_struct<0, params...> {
  static const char* format_string() {
    return concat<base_format_string_struct::format_string, params...>::value;
  }
};

Demo

But I would probably go with constexpr functions:

#if 1 // missing some constexpr for std::array/std::copy

template <typename T, std::size_t N>
struct my_array
{
    T data[N];
};

template <typename CIT, typename IT>
constexpr void copy(CIT begin, CIT end, IT dest)
{
    for (CIT it = begin; it != end;   it)
    {
        *dest = *it;
          dest;
    }
}
#endif

template <std::size_t N>
constexpr my_array<char, 2   8 * N   1> format_string_struct_impl()
{
    my_array<char, 2   8 * N   1> res{};
    constexpr char init[] = "{}"; // size == 2
    constexpr char extra[] = R"( {}="{}")"; // size == 8
    copy(init, init   2, res.data);
    for (std::size_t i = 0; i != N;   i) {
        copy(extra, extra   8, res.data   2   i * 8);
    }
    return res;
}

template <std::size_t N>
constexpr my_array<char, 2   8 * N   1> format_string_struct()
{
    // Ensure the computation is done compile time.
    constexpr auto res = format_string_struct_impl<N>();
    return res;
}

Demo