20 General utilities library [utilities]

20.20 Formatting [format]

20.20.5 Formatter [format.formatter] Class template basic_­format_­context [format.context]

namespace std {
  template<class Out, class charT>
  class basic_format_context {
    basic_format_args<basic_format_context> args_;      // exposition only
    Out out_;                                           // exposition only

    using iterator = Out;
    using char_type = charT;
    template<class T> using formatter_type = formatter<T, charT>;

    basic_format_arg<basic_format_context> arg(size_t id) const;
    std::locale locale();

    iterator out();
    void advance_to(iterator it);
An instance of basic_­format_­context holds formatting state consisting of the formatting arguments and the output iterator.
Out shall model output_­iterator<const charT&>.
format_­context is an alias for a specialization of basic_­format_­context with an output iterator that appends to string, such as back_­insert_­iterator<string>.
Similarly, wformat_­context is an alias for a specialization of basic_­format_­context with an output iterator that appends to wstring.
For a given type charT, implementations are encouraged to provide a single instantiation of basic_­format_­context for appending to basic_­string<charT>, vector<charT>, or any other container with contiguous storage by wrapping those in temporary objects with a uniform interface (such as a span<charT>) and polymorphic reallocation.
— end note]
basic_format_arg<basic_format_context> arg(size_t id) const;
Returns: args_­.get(id).
std::locale locale();
Returns: The locale passed to the formatting function if the latter takes one, and std​::​locale() otherwise.
iterator out();
Returns: out_­.
void advance_to(iterator it);
Effects: Equivalent to: out_­ = it;
struct S { int value; };

template<> struct std::formatter<S> {
  size_t width_arg_id = 0;

  // Parses a width argument id in the format { digit }.
  constexpr auto parse(format_parse_context& ctx) {
    auto iter = ctx.begin();
    auto get_char = [&]() { return iter != ctx.end() ? *iter : 0; };
    if (get_char() != '{')
      return iter;
    char c = get_char();
    if (!isdigit(c) || (++iter, get_char()) != '}')
      throw format_error("invalid format");
    width_arg_id = c - '0';
    return ++iter;

  // Formats an S with width given by the argument width_­arg_­id.
  auto format(S s, format_context& ctx) {
    int width = visit_format_arg([](auto value) -> int {
      if constexpr (!is_integral_v<decltype(value)>)
        throw format_error("width is not integral");
      else if (value < 0 || value > numeric_limits<int>::max())
        throw format_error("invalid width");
        return value;
      }, ctx.arg(width_arg_id));
    return format_to(ctx.out(), "{0:x<{1}}", s.value, width);

std::string s = std::format("{0:{1}}", S{42}, 10);  // value of s is "xxxxxxxx42"
— end example]