Numeric grouping and locales

fmt is locale-independent by default, and that is a feature, not an oversight — you opt in to locale-dependent output exactly where a human is going to read it, and nowhere else.

The locale specifier

The L flag applies the active locale's digit grouping and decimal separator to a number.

fmt::format(std::locale("en_US.UTF-8"), "{:L}", 1234567);   // "1,234,567"
fmt::format(std::locale("de_DE.UTF-8"), "{:L}", 1234567.5);  // "1.234.567,5"

Default is locale-independent

Locale-independence by default is why fmt output is safe to write to files, wire formats and logs

fmt::format("{}", 1234567) always produces 1234567, regardless of what locale the process has set globally. That's exactly what you want for anything machine-read back later — a CSV column, a JSON number, a log line a parser scans.

The trap

A global locale change silently alters every locale-aware format — including one in a file your parser reads back

std::locale::global(...) affects every subsequent {:L} format in the process. A library that changes the global locale for its own reasons can silently break formatting elsewhere in the same binary that happens to use L. Prefer passing an explicit std::locale to the call site over relying on whatever the global locale happens to be.

Comparison table

	printf	iostreams	fmt
Default locale sensitivity	Locale-dependent (printf respects the C locale)	Locale-dependent (stream's imbued locale)	Locale-independent
Opting in	setlocale — process-global	stream.imbue(locale) — per stream	{:L} with an explicit or global std::locale — per call
Opting out	Reset with setlocale(LC_ALL, "C")	stream.imbue(std::locale::classic())	Default behavior — nothing to opt out of

Availability

Locale support lives behind <fmt/format.h> and pulls in <locale>. Embedded and other size-constrained builds sometimes disable full locale support and fall back to a compile-time-fixed thousands separator via FMT_STATIC_THOUSANDS_SEPARATOR.

Check whether your target even has locale support before relying on the L specifier

Some embedded toolchains ship a minimal C library without locale facilities at all. If {:L} is load bearing, confirm it on the actual target toolchain, not just on desktop.

See also

• Sign and numeric precision — the numeric formatting L builds on.
• Format spec syntax — where L sits in the full grammar.
• Unicode and encoding notes — the other place locale-adjacent surprises show up.
• fmt overview — the full doc set.