User-defined Types

To format a user-defined type, I have to specialize the class std::formatter for my user-defined type. This means, in particular, I have to implement the member functions parse and format.

• parse:

  • Accepts the parse context
  • Parses the parse context
  • Returns an iterator to the end of the format specification
  • Throws a std::format_error in case of an error

• format:

  • Gets the value t, which should be formatted, and the format context fc
  • Formats t according to the format context
  • Writes the output to fc.out()
  • Returns an iterator that represents the end of the output

Let me put the theory into practice and format a std::vector.

Formatting a std::vector

My first specialization of the class std::formatter is as easy as possible. I specify a format specification used for each element of the container.