multiple formats for a single type ?

[jcelerier]

So, this is more or less a follow-up of #859 : how can there be multiple distinct ways to format a given type with fmt 5 ?

e.g., I'd like to have in one file a formatter which generates a debug-like output, and in another file a formatter which generates some HTML representation, for a given C++ type foo.

With fmt 4 this is easy: just have a function format_html and another format_debug which both take a fmt::MemoryWriter and a foo but serialize the type according to the desired output.

[vitaut]

With fmt 5 you can also have two functions that perform different formatting, e.g.

// Alternative you can format into memory_buffer instead of returning a string.
std::string format_xml(int n) {
  return fmt::format("<answer>{}</answer>", n);
}

std::string format_debug(int n) {
  return fmt::format("answer={}", n);
}

[jcelerier]

but what if a user-defined formatter needs different ways to be formatted ?

how would you format, e.g. a std::vector<int> to [ 1, 2, 3 ] in one function, and <list><value>1</value><value>2</value><value>3</value></list> in another ? fmt::formatter can only be specialized once.

[jcelerier]

@vitaut this really doesn't work: for instance I cannot provide a custom serialization for std::vector :

template <typename T, typename Alloc>
struct formatter<std::vector<T, Alloc>>
{
  template <typename ParseContext>
  constexpr auto parse(ParseContext &ctx) { return ctx.begin(); }

  template <typename FormatContext>
  auto format(const std::vector<T, Alloc>& v, FormatContext &ctx)
  {
    auto out = ctx.out();
    format_to(out, "[");
    const auto n = v.size();
    if (n > 0)
    {
      format_to(out, "{}", v[0]);

      for (std::size_t i = 1; i < n; i++)
      {
        format_to(out, ", {}", v[i]);
      }
    }
    format_to(out, "]");
    return ctx.out();
  }
};

because if at some point someone includes <fmt/ranges.h> it won't compile anymore

[jcelerier]

(which is sensibly the same problem than #865)

[vitaut]

but what if a user-defined formatter needs different ways to be formatted ?

fmt provides you with formatting primitives that you can use to build your own functions that write data in XML or any other format. You probably shouldn't plug the data-format specific logic into the fmt's formatters themselves in the same way as you wouldn't write a custom ostream class for XML output.

The problem with ranges.h is a separate issue that will be addressed in #865.