What are the differences between libfmt and std::format?

Question

I am aware that the c++20 format proposal is a formalization of parts of libfmt, and that libfmt is a compliant implementation of that formalization. However, it's my understanding that libfmt provides additional functionality beyond that specified in the c++20 standard. What are the additional features?

Additional, are the major compiler vendors simply including a subset of libfmt or reimplementing it?

Aspasia

Answer 1

There are a bunch of things in libfmt that are not in C++20 format:

• fmt::print() to print directly to stdout.
• fmt::memory_buffer as basically a dynamically sized container that you could format into via fmt::format_to(buf, ...).
• Support for formatting ranges and tuples, including fmt::join().
• Support for named arguments like fmt::print("Elapsed time: {s:.2f} seconds", "s"_a=1.23);
• Compile-time format strings via FMT_COMPILE, although both fmt::format and C++20's std::format do compile-time format string parsing by default. fmt::format has an escape hook for this named fmt::runtime, std::format has no such hook.

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C++23 update: std::print (P2093) and formatting ranges and tuples (P2286) are in C++23.

Harts

I don't think fmt::color and fmt::emphasis is in C++20 either, but I'd love to be wrong. :-) – Magdalen

Does C++20 allow compile-time string_view literals as a substitute for FMT_COMPILE? – Coffeepot

Or put another way, could C++20 implementations perform the same sort of optimizations as FMT_COMPILE when a string_view literal is passed as the format string? – Coffeepot