README.md
{fmt} is an open-source formatting library providing a fast and safe alternative to C stdio and C++ iostreams.
Q&A: ask questions on StackOverflow with the tag fmt.
Try {fmt} in Compiler Explorer.
(s)printf, iostreams, to_string and
to_chars, see Speed tests and Converting a
hundred million integers to strings per
secondbase.h, format.h
and format-inl.h, and compiled code; see Compile time and code
bloat-Wall -Wextra -pedanticFMT_HEADER_ONLY macroSee the documentation for more details.
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:
| 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:
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.
Please refer to Building the library for instructions on how to build the library and run the unit tests.
Benchmarks reside in a separate repository, format-benchmarks, so to run the benchmarks you first need to clone this repository and generate Makefiles with CMake:
$ git clone --recursive https://github.com/fmtlib/format-benchmark.git
$ cd format-benchmark
$ cmake .
Then you can run the speed test:
$ make speed-test
or the bloat test:
$ make bloat-test
clang-tidy v18 provides the
modernize-use-std-print
check that is capable of converting occurrences of printf and
fprintf to fmt::print if configured to do so. (By default it
converts to std::print.)
If you are aware of other projects using this library, please let me know by email or by submitting an issue.
The {fmt} library is maintained by Victor Zverovich (vitaut) with contributions from many other people. See Contributors and Releases for some of the names. Let us know if your contribution is not listed or mentioned incorrectly and we'll make it right.