Optimized buffer::append, resulting in up to ~16% improvement on spdlog benchmarks (#4541). Thanks @fyrsta7.

Worked around an ABI incompatibility in std::locale_ref between clang and gcc (#4573).

Made std::variant and std::expected formatters work with format_as (#4574, #4575). Thanks @phprus.

Made fmt::join<string_view> work with C++ modules (#4379, #4577). Thanks @Arghnews.

Exported fmt::is_compiled_string and operator""_cf from the module (#4544). Thanks @CrackedMatter.

Fixed a compatibility issue with C++ modules in clang (#4548). Thanks @tsarn.

Added support for cv-qualified types to the std::optional formatter (#4561, #4562). Thanks @OleksandrKvl.

Added demangling support (used in exception and std::type_info formatters) for libc++ and clang-cl (#4542, #4560, #4568, #4571). Thanks @FatihBAKIR and @rohitsutreja.

Switched to global malloc/free to enable allocator customization (#4569, #4570). Thanks @rohitsutreja.

Made the FMT_USE_CONSTEVAL macro configurable by users (#4546). Thanks @SnapperTT.

Fixed compilation with locales disabled in the header-only mode (#4550).

Fixed compilation with clang 21 and -std=c++20 (#4552).

Fixed a dynamic linking issue with clang-cl (#4576, #4584). Thanks @FatihBAKIR.

Fixed a warning suppression leakage on gcc (#4588). Thanks @ZedThree.

Made more internal color APIs constexpr (#4581). Thanks @ishani.

Fixed compatibility with clang as a host compiler for NVCC (#4564). Thanks @valgur.

Fixed various warnings and lint issues (#4565, #4572, #4557). Thanks @LiangHuDream and @teruyamato0731.

Improved documentation (#4549, #4551, #4566, #4567, #4578,). Thanks @teruyamato0731, @petersteneteg and @zimmerman-dev.

Optimized the default floating point formatting (#3675, #4516). In particular, formatting a double with format string compilation into a stack allocated buffer is more than 60% faster in version 12.0 compared to 11.2 according to dtoa-benchmark:

Function  Time (ns)  Speedup
fmt11        34.471    1.00x
fmt12        21.000    1.64x

Added constexpr support to fmt::format. For example:

#include <fmt/compile.h>

using namespace fmt::literals;
std::string s = fmt::format(""_cf, 42);

now works at compile time provided that std::string supports constexpr (#3403, #4456). Thanks @msvetkin.

Added FMT_STATIC_FORMAT that allows formatting into a string of the exact required size at compile time.

For example:

#include <fmt/compile.h>

constexpr auto s = FMT_STATIC_FORMAT("{}", 42);

compiles to just

__ZL1s:
      .asciiz "42"

It can be accessed as a C string with s.c_str() or as a string view with s.str().

Improved C++20 module support (#4451, #4459, #4476, #4488, #4491, #4495). Thanks @arBmind, @tkhyn, @Mishura4, @anonymouspc and @autoantwort.

Switched to using estimated display width in precision. For example:

fmt::print("|{:.4}|\n|1234|\n", "🐱🐱🐱");

prints

because 🐱 has an estimated width of 2 (#4272, #4443, #4475). Thanks @nikhilreddydev and @localspook.

Fix interaction between debug presentation, precision, and width for strings (#4478). Thanks @localspook.

Implemented allocator propagation on basic_memory_buffer move (#4487, #4490). Thanks @toprakmurat.

Fixed an ambiguity between std::reference_wrapper<T> and format_as formatters (#4424, #4434). Thanks @jeremy-rifkin.

Removed the following deprecated APIs:

• has_formatter: use is_formattable instead, - basic_format_args::parse_context_type, basic_format_args::formatter_type and similar aliases in context types, - wide stream overload of fmt::printf, - wide stream overloads of fmt::print that take text styles, - is_*char traits, - fmt::localtime.

Deprecated wide overloads of fmt::fprintf and fmt::sprintf.

Improved diagnostics for the incorrect usage of fmt::ptr (#4453). Thanks @TobiSchluter.

Made handling of ANSI escape sequences more efficient (#4511, #4528). Thanks @localspook and @Anas-Hamdane.

Fixed a buffer overflow on all emphasis flags set (#4498). Thanks @dominicpoeschko.

Fixed an integer overflow for precision close to the max int value.

Fixed compatibility with WASI (#4496, #4497). Thanks @whitequark.

Fixed back_insert_iterator detection, preventing a fallback on slower path that handles arbitrary iterators (#4454).

Fixed handling of invalid glibc FILE buffers (#4469).

Added wchar_t support to the std::byte formatter (#4479, #4480). Thanks @phprus.

Changed component prefix from fmt- to fmt_ for compatibility with NSIS/CPack on Windows, e.g. fmt-doc changed to fmt_doc (#4441, #4442). Thanks @n-stein.

Added the FMT_CUSTOM_ASSERT_FAIL macro to simplify providing a custom fmt::assert_fail implementation (#4505). Thanks @HazardyKnusperkeks.

Switched to FMT_THROW on reporting format errors so that it can be overriden by users when exceptions are disabled (#4521). Thanks @HazardyKnusperkeks.

Improved master project detection and disabled install targets when using {fmt} as a subproject by default (#4536). Thanks @crueter.

Made various code improvements (#4445, #4448, #4473, #4522). Thanks @localspook, @tchaikov and @way4sahil.

Added Conan instructions to the docs (#4537). Thanks @uilianries.

Removed Bazel files to avoid issues with downstream packaging (#4530). Thanks @mering.

Added more entries for generated files to .gitignore (#4355, #4512). Thanks @dinomight and @localspook.

Fixed various warnings and compilation issues (#4447, #4470, #4474, #4477, #4471, #4483, #4515, #4533, #4534). Thanks @dodomorandi, @localspook, @remyjette, @Tomek-Stolarczyk, @Mishura4, @mattiasljungstrom and @FatihBAKIR.

Added the s specifier for std::error_code. It allows formatting an error message as a string. For example:

#include <fmt/std.h>

int main() {
  auto ec = std::make_error_code(std::errc::no_such_file_or_directory);
  fmt::print("{:s}\n", ec);
}

prints

No such file or directory

(The actual message is platform-specific.)

Fixed formatting of std::chrono::local_time and tm (#3815, #4350). For example (godbolt):

#include <fmt/chrono.h>

int main() {
  std::chrono::zoned_time zt(
    std::chrono::current_zone(),
    std::chrono::system_clock::now());
  fmt::print("{}", zt.get_local_time());
}

is now formatted consistenly across platforms.

Added diagnostics for cases when timezone information is not available. For example:

fmt::print("{:Z}", std::chrono::local_seconds());

now gives a compile-time error.

Deprecated fmt::localtime in favor of std::localtime.

Fixed compilation with GCC 15 and C++20 modules enabled (#4347). Thanks @tkhyn.

Fixed handling of named arguments in format specs (#4360, #4361). Thanks @dinomight.

Added error reporting for duplicate named arguments (#4367). Thanks @dinomight.

Fixed formatting of long with FMT_BUILTIN_TYPES=0 (#4375, #4394).

Optimized text_style using bit packing (#4363). Thanks @localspook.

Added support for incomplete types (#3180, #4383). Thanks @localspook.

Fixed a flush issue in fmt::print when using libstdc++ (#4398).

Fixed fmt::println usage with FMT_ENFORCE_COMPILE_STRING and legacy compile-time checks (#4407). Thanks @madmaxoft.

Removed legacy header fmt/core.h from docs (#4421, #4422). Thanks @krzysztofkortas.

Worked around limitations of __builtin_strlen during constant evaluation (#4423, #4429). Thanks @brevzin.

Worked around a bug in MSVC v141 (#4412, #4413). Thanks @hirohira9119.

Removed the fmt_detail namespace (#4324).

Removed specializations of std::is_floating_point in tests (#4417).

Fixed a CMake error when setting CMAKE_MODULE_PATH in the pedantic mode (#4426). Thanks @rlalik.

Updated the Bazel config (#4400). Thanks @Vertexwahn.

Fixed ABI compatibility with earlier 11.x versions on Windows (#4359).

Improved the logic of switching between fixed and exponential format for float (#3649).

Moved is_compiled_string to the public API (#4342). Thanks @SwooshyCueb.

Simplified implementation of operator""_cf (#4349). Thanks @localspook.

Fixed __builtin_strlen detection (#4329). Thanks @localspook.

Fixed handling of BMI paths with the Ninja generator (#4344). Thanks @tkhyn.

Fixed gcc 8.3 compile errors (#4331, #4336). Thanks @sergiud.

Fixed a bogus MSVC warning (#4356). Thanks @dinomight.

Fixed compilation on GCC 9.4 (#4313).

Worked around an internal compiler error when using C++20 modules with GCC 14.2 and earlier (#4295).

Worked around a bug in GCC 6 (#4318).

Fixed an issue caused by instantiating formatter<const T> (#4303, #4325). Thanks @timsong-cpp.

Fixed formatting into std::ostreambuf_iterator when using format string compilation (#4309, #4312). Thanks @phprus.

Restored a constraint on the map formatter so that it correctly reports as unformattable when the element is (#4326). Thanks @timsong-cpp.

Reduced the size of format specs (#4298).

Readded args() to fmt::format_context (#4307, #4310). Thanks @Erroneous1.

Fixed a bogus MSVC warning (#4314, #4322). Thanks @ZehMatt.

Fixed a pedantic mode error in the CMake config (#4327). Thanks @rlalik.

Fixed ABI compatibility with earlier 11.x versions (#4292).

Added wchar_t support to the std::bitset formatter (#4285, #4286, #4289, #4290). Thanks @phprus.

Prefixed CMake components with fmt- to simplify usage of {fmt} via add_subdirectory (#4283).

Updated docs for meson (#4291). Thanks @trim21.

Fixed a compilation error in chrono on nvcc (#4297, #4301). Thanks @breyerml.

Fixed various warnings (#4288, #4299). Thanks @GamesTrap and @edo9300.

Fixed ABI compatibility with earlier 11.x versions (#4278).

Defined CMake components (core and doc) to allow docs to be installed separately (#4276). Thanks @carlsmedstad.

Improved C++20 module support (#4081, #4083, #4084, #4152, #4153, #4169, #4190, #4234, #4239). Thanks @kamrann and @Arghnews.

Reduced debug (unoptimized) binary code size and the number of template instantiations when passing formatting arguments. For example, unoptimized binary code size for fmt::print("{}", 42) was reduced by ~40% on GCC and ~60% on clang (x86-64).

GCC:

• Before: 161 instructions of which 105 are in reusable functions (godbolt).
• After: 116 instructions of which 60 are in reusable functions (godbolt).

Clang:

• Before: 310 instructions of which 251 are in reusable functions (godbolt).
• After: 194 instructions of which 135 are in reusable functions (godbolt).

Added an experimental fmt::writer API that can be used for writing to different destinations such as files or strings (#2354). For example (godbolt):

#include <fmt/os.h>

void write_text(fmt::writer w) {
  w.print("The answer is {}.", 42);
}

int main() {
  // Write to FILE.
  write_text(stdout);

  // Write to fmt::ostream.
  auto f = fmt::output_file("myfile");
  write_text(f);

  // Write to std::string.
  auto sb = fmt::string_buffer();
  write_text(sb);
  std::string s = sb.str();
}

Added width and alignment support to the formatter of std::error_code.

Made std::expected<void, E> formattable (#4145, #4148). For example (godbolt):

fmt::print("{}", std::expected<void, int>());

prints

expected()

Thanks @phprus.

Made fmt::is_formattable<void> SFINAE-friendly (#4147).

Added support for _BitInt formatting when using clang (#4007, #4072, #4140, #4173, #4176). For example (godbolt):

using int42 = _BitInt(42);
fmt::print("{}", int42(100));

Thanks @Arghnews.

Added the n specifier for tuples and pairs (#4107). Thanks @someonewithpc.

Added support for tuple-like types to fmt::join (#4226, #4230). Thanks @phprus.

Made more types formattable at compile time (#4127). Thanks @AnthonyVH.

Implemented a more efficient compile-time fmt::formatted_size (#4102, #4103). Thanks @phprus.

Fixed compile-time formatting of some string types (#4065). Thanks @torshepherd.

Made compiled version of fmt::format_to work with std::back_insert_iterator<std::vector<char>> (#4206, #4211). Thanks @phprus.

Added a formatter for std::reference_wrapper (#4163, #4164). Thanks @yfeldblum and @phprus.

Added experimental padding support (glibc strftime extension) to %m, %j and %Y (#4161). Thanks @KKhanhH.

Made microseconds formatted as us instead of µs if the Unicode support is disabled (#4088).

Fixed an unreleased regression in transcoding of surrogate pairs (#4094, #4095). Thanks @phprus.

Made fmt::appender satisfy std::output_iterator concept (#4092, #4093). Thanks @phprus.

Made std::iterator_traits<fmt::appender> standard-conforming (#4185). Thanks @CaseyCarter.

Made it easier to reuse fmt::formatter<std::string_view> for types with an implicit conversion to std::string_view (#4036, #4055). Thanks @Arghnews.

Made it possible to disable <filesystem> use via FMT_CPP_LIB_FILESYSTEM for compatibility with some video game console SDKs, e.g. Nintendo Switch SDK (#4257, #4258, #4259). Thanks @W4RH4WK and @phprus.

Fixed compatibility with platforms that use 80-bit long double (#4245, #4246). Thanks @jsirpoma.

Added support for UTF-32 code units greater than 0xFFFF in fill (#4201).

Fixed handling of legacy encodings on Windows with GCC (#4162).

Made fmt::to_string take fmt::basic_memory_buffer by const reference (#4261, #4262). Thanks @sascha-devel.

Added fmt::dynamic_format_arg_store::size (#4270). Thanks @hannes-harnisch.

Removed the ability to control locale usage via an undocumented FMT_STATIC_THOUSANDS_SEPARATOR in favor of FMT_USE_LOCALE.

Renamed FMT_EXCEPTIONS to FMT_USE_EXCEPTIONS for consistency with other similar macros.

Improved include directory ordering to reduce the chance of including incorrect headers when using multiple versions of {fmt} (#4116). Thanks @cdzhan.

Made it possible to compile a subset of {fmt} without the C++ runtime.

Improved documentation and README (#4066, #4117, #4203, #4235). Thanks @zyctree and @nikola-sh.

Improved the documentation generator (#4110, #4115). Thanks @rturrado.

Improved CI (#4155, #4151). Thanks @phprus.

Fixed various warnings and compilation issues (#2708, #4091, #4109, #4113, #4125, #4129, #4130, #4131, #4132, #4133, #4144, #4150, #4158, #4159, #4160, #4170, #4177, #4187, #4188, #4194, #4200, #4205, #4207, #4208, #4210, #4220, #4231, #4232, #4233, #4236, #4267, #4271). Thanks @torsten48, @Arghnews, @tinfoilboy, @aminya, @Ottani, @zeroomega, @c4v4, @kongy, @vinayyadav3016, @sergio-nsk, @phprus and @YexuanXiao.

Fixed compatibility with non-POSIX systems (#4054, #4060).

Fixed performance regressions when using std::back_insert_iterator with fmt::format_to (#4070).

Fixed handling of std::generator and move-only iterators (#4053, #4057). Thanks @Arghnews.

Made formatter<std::string_view>::parse work with types convertible to std::string_view (#4036, #4055). Thanks @Arghnews.

Made volatile void* formattable (#4049, #4056). Thanks @Arghnews.

Made Glib::ustring not be confused with std::string (#4052).

Made fmt::context iterator compatible with STL algorithms that rely on iterator category (#4079).

Fixed version number in the inline namespace (#4047).

Fixed disabling Unicode support via CMake (#4051).

Fixed deprecated visit_format_arg (#4043). Thanks @nebkat.

Fixed handling of a sign and improved the std::complex formater (#4034, #4050). Thanks @tesch1 and @phprus.

Fixed ADL issues in fmt::printf when using C++20 (#4042). Thanks @toge.

Removed a redundant check in the formatter for std::expected (#4040). Thanks @phprus.