{fmt} is an open-source formatting library providing a fast and safe alternative to C stdio and C++ iostreams.

Documentation

Cheat Sheets

Q&A: ask questions on StackOverflow with the tag fmt.

Try {fmt} in Compiler Explorer.

Features

• Simple format API with positional arguments for localization
• Implementation of C++20 std::format and C++23 std::print
• Format string syntax similar to Python's format
• Fast IEEE 754 floating-point formatter with correct rounding, shortness and round-trip guarantees using the Dragonbox algorithm
• Portable Unicode support
• Safe printf implementation including the POSIX extension for positional arguments
• Extensibility: support for user-defined types
• High performance: faster than common standard library implementations of (s)printf, iostreams, to_string and to_chars, see Speed tests and Converting a hundred million integers to strings per second
• Small code size both in terms of source code with the minimum configuration consisting of just three files, core.h, format.h and format-inl.h, and compiled code; see Compile time and code bloat
• Reliability: the library has an extensive set of tests and is continuously fuzzed
• Safety: the library is fully type-safe, errors in format strings can be reported at compile time, automatic memory management prevents buffer overflow errors
• Ease of use: small self-contained code base, no external dependencies, permissive MIT license
• Portability with consistent output across platforms and support for older compilers
• Clean warning-free codebase even on high warning levels such as -Wall -Wextra -pedantic
• Locale independence by default
• Optional header-only configuration enabled with the FMT_HEADER_ONLY macro

See the documentation for more details.

Examples

Print to stdout (run)

#include <fmt/core.h>

int main() {
  fmt::print("Hello, world!\n");
}

Format a string (run)

std::string s = fmt::format("The answer is {}.", 42);
// s == "The answer is 42."

Format a string using positional arguments (run)

std::string s = fmt::format("I'd rather be {1} than {0}.", "right", "happy");
// s == "I'd rather be happy than right."

Print dates and times (run)

#include <fmt/chrono.h>

int main() {
  auto now = std::chrono::system_clock::now();
  fmt::print("Date and time: {}\n", now);
  fmt::print("Time: {:%H:%M}\n", now);
}

Output:

Date and time: 2023-12-26 19:10:31.557195597
Time: 19:10

Print a container (run)

#include <vector>
#include <fmt/ranges.h>

int main() {
  std::vector<int> v = {1, 2, 3};
  fmt::print("{}\n", v);
}

Output:

[1, 2, 3]

Check a format string at compile time

std::string s = fmt::format("{:d}", "I am not a number");

This gives a compile-time error in C++20 because d is an invalid format specifier for a string.

Write a file from a single thread

#include <fmt/os.h>

int main() {
  auto out = fmt::output_file("guide.txt");
  out.print("Don't {}", "Panic");
}

This can be up to 9 times faster than fprintf.

Print with colors and text styles

#include <fmt/color.h>

int main() {
  fmt::print(fg(fmt::color::crimson) | fmt::emphasis::bold,
             "Hello, {}!\n", "world");
  fmt::print(fg(fmt::color::floral_white) | bg(fmt::color::slate_gray) |
             fmt::emphasis::underline, "Olá, {}!\n", "Mundo");
  fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic,
             "你好{}！\n", "世界");
}

Output on a modern terminal with Unicode support:

Performance

{fmt} can be tens of percent to 20–30 times faster than sprintf and iostreams, especially for numeric formatting. It minimizes dynamic memory allocations and can optionally compile format strings into efficient formatting code.

See format-benchmark and dtoa-benchmark for benchmarks and methodology.

Time per double (smaller is better):

ostringstream and sprintf are omitted because they are an order of magnitude slower than the other methods.

Compile time and code bloat

The script bloat-test.py from format-benchmark measures the compile-time and code-size overhead each formatting method adds to application code. It generates 100 translation units and uses printf or its alternative five times in each to simulate a medium-sized project. Library and module build costs are excluded. Results on an Apple M5 Max running macOS 26.6.2 with Apple Clang 21.0.0 (clang-2100.1.1.101), taking the best of three runs, are shown in the following tables.

Optimized build (-O3)

Method	Compile time, s	Binary size, KiB	Stripped size, KiB
printf	1.6	54	50
IOStreams	25.5	98	84
fmt 12.2 (headers)	5.1	54	50
fmt 12.2 (module)	3.7	59	50
Boost Format 1.92	49.1	517	317

Using modular {fmt} reduces optimized application-code compile time by 27% without changing the reported stripped binary size.

Non-optimized build

Method	Compile time, s	Binary size, KiB	Stripped size, KiB
printf	1.6	54	50
IOStreams	26.0	88	68
fmt 12.2 (headers)	4.9	87	84
fmt 12.2 (module)	3.2	77	68
Boost Format 1.92	35.7	741	431

libc, libc++, libfmt, and libfmt-module were linked as shared libraries to compare formatting function overhead only. Boost Format is header-only.

Projects using {fmt}

Notable users include:

• Apple's FoundationDB
• Blizzard Battle.net
• Ceph
• ClickHouse
• Envoy
• Folly
• MariaDB
• MongoDB
• PyTorch
• Quill
• Seastar
• spdlog
• Windows Terminal

Find more projects using {fmt} on GitHub.