Truncate marker for strings

[mordante]

When using the precision for string output the string may get truncated. It would be great if the output has a marker indicating the output is truncated. I created a small proof of concept patch that provides this behaviour [1]. The patch is against 5.3.0 in a local project so the git index is bogus. It modifies the following syntax:
["." precision]
to
["." precision["," truncate]]
Where truncate is a single character. That means the following code:
fmt::print(FMT_STRING("{:.5}\n{:.5,#}\n"), "foobar", "foobar");
will output

fooba
foob#

The patch doesn't work with the { arg_id } precision syntax. I actually would like the ellipse …, but that seems related to #1109.

Is this behaviour wanted and would this be the proper approach?

[1] https://filebin.ca/4eMhuevLFfia/truncate.patch

[vitaut]

Thanks for the suggestions. I would rather not complicate the format specifications for such a relatively narrow use case but you can implement this using the extension API without changing the core library by wrapping a string in some type:

struct trunc {
  std::string_view s;
};

struct fmt::formatter<trunc> {
  // Implement custom parsing and formatting with truncation marker.
};

fmt::print(FMT_STRING("{:.5}\n{:.5,#}\n"), "foobar", trunc("foobar"));

Truncation marker can either be part of a format string or an argument to trunc. This will also work with named arguments.

[mordante]

Thanks for suggesting an alternative way to get this done.