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

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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, base.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/base.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:

Benchmarks

Speed tests

| Library | Method | Run Time, s | |-------------------|---------------|-------------| | libc | printf | 0.66 | | libc++ | std::ostream | 1.63 | | {fmt} 12.1 | fmt::print | 0.44 | | Boost Format 1.88 | boost::format | 3.89 | | Folly Format | folly::format | 1.28 |

{fmt} is the fastest of the benchmarked methods, ~50% faster than printf.

The above results were generated by building tinyformat_test.cpp on macOS 15.6.1 with clang++ -O3 -DNDEBUG -DSPEED_TEST -DHAVE_FORMAT, and taking the best of three runs. In the test, the format string "%0.10f:%04d:%+g:%s:%p:%c:%%\n" or equivalent is filled 2,000,000 times with output sent to /dev/null; for further details refer to the source.

{fmt} is up to 20-30x faster than std::ostringstream and sprintf on IEEE754 float and double formatting (dtoa-benchmark) and faster than double-conversion and ryu:

Compile time and code bloat

The script bloat-test.py from format-benchmark tests compile time and code bloat for nontrivial projects. It generates 100 translation units and uses printf() or its alternative five times in each to simulate a medium-sized project. The resulting executable size and compile time (Apple clang version 15.0.0 (clang-1500.1.0.2.5), macOS Sonoma, best of three) is shown in the following tables.

Optimized build (-O3)

| Method | Compile Time, s | Executable size, KiB | Stripped size, KiB | |-----------------|-----------------|----------------------|--------------------| | printf | 1.6 | 54 | 50 | | IOStreams | 28.4 | 98 | 84 | | {fmt} 1122268 | 5.0 | 54 | 50 | | tinyformat | 32.6 | 164 | 136 | | Boost Format | 55.0 | 530 | 317 |

{fmt} is fast to compile and is comparable to printf in terms of per-call binary size (within a rounding error on this system).

Non-optimized build

| Method | Compile Time, s | Executable size, KiB | Stripped size, KiB | |-----------------|-----------------|----------------------|--------------------| | printf | 1.4 | 54 | 50 | | IOStreams | 27.0 | 88 | 68 | | {fmt} 1122268 | 4.7 | 87 | 84 | | tinyformat | 28.1 | 185 | 145 | | Boost Format | 38.9 | 678 | 381 |

libc, lib(std)c++, and libfmt are all linked as shared libraries to compare formatting function overhead only. Boost Format is a header-only library so it doesn't provide any linkage options.

Running the tests

Please refer to Building the library for instructions on how to build the lib