QtCS2024 std::format

Session Summary

In this session we'll discuss std::format and its support in Qt.

We'll try to discuss sevaral main questions:

• Which Qt types should support formattings with std::format?
• Do we limit ourselves with the standard format specifiers, or do we allow custom specifiers?
• Implementation details

Bonus topic:

Do we need an alternative to QString::arg() with std::format-like syntax?

Session Owners

Ivan Note taker: Thiago

Notes

• We would like to have support for std::format for our printable types (already supported in QDebug) so users don't have to define them locally
  • would clash later if we decided to add them
• Does it make sense for all types?
  • QDebug can print almost anything but doesn't guarantee what it will print (it just has to be useful)
  • Should std::format reuse all of that? Should we guarantee what the output will be and retain it? Might make it impossible to use QDebug instead.
• Marc suggests instead focusing on types that have equivalents in the Standard Library and document their behaviour & flags
  • Strings, floats, containers, geometric types, date/time
  • We gain experience in the process
  • We can reuse the QDebug code on a case-by-case basis. We can revisit the decision to use QDebug throughout later.
• Can the output be lossy?
• Headers: <format> is somewhat heavy, we shouldn't #include everywhere
  • But this means users wouldn't know the formatters exist and may go on to define their own implementatoins
  • Is this a big deal? Would users not realise that the code exist?
  • Suggestion: we put it in the classes' own headers and add a #define to disable the content, to improve compilation times
    • May not help, because the implementation of (for example) <chrono%gt; may bring in std::format

Formatting to QString

The Standard Library seems to be designed so we can provide our own destination format. Requires exploration.

Misc

Should we have a qDebug with std::format-style patterns?

Trivia: why does QTest's QCOMPARE print C++-escaped strings? So you can QCOMPARE to a dummy string, then copy the output verbatim to your source code.