C++ Format

C++ Format is an open-source formatting library for C++. It can be used as a safe alternative to printf or as a fast alternative to IOStreams.

Features

• Two APIs: faster concatenation-based write API and slower (but still very fast) replacement-based format API with positional arguments for localization.
• Write API similar to the one used by IOStreams but much faster and more consistent.
• Format API with format string syntax similar to the one used by str.format in Python.
• Support for user-defined types.
• High speed: performance of the format API is close to that of glibc's printf and better than performance of IOStreams. See Speed tests and Fast integer to string conversion in C++.
• Small code size both in terms of source code (format consists of a single header file and a single source file) and compiled code. See Compile time and code bloat.
• Reliability: the library has an extensive set of unit tests.
• Safety: the library is fully type safe, errors in format strings are reported using exceptions, automatic memory management prevents buffer overflow errors.
• Ease of use: small self-contained code base, no external dependencies, permissive BSD license.
• Portability with consistent output across platforms and support for older compilers.
• Clean warning-free codebase even on high warning levels (-Wall -Wextra -pedantic).
• Support for wide strings.

See the documentation for more details.

Examples

This prints Hello, world! to stdout:

fmt::print("Hello, {}!", "world");  // uses Python-like format
fmt::printf("Hello, %s!", "world"); // uses printf format

Arguments can be accessed by position and arguments' indices can be repeated:

std::string s = fmt::format("{0}{1}{0}", "abra", "cad");
// s == "abracadabra"

C++ Format can be used as a safe portable replacement for itoa:

fmt::Writer w;
w << 42;           // replaces itoa(42, buffer, 10)
w << fmt::hex(42); // replaces itoa(42, buffer, 16)
// access the string using w.str() or w.c_str()

An object of any user-defined type for which there is an overloaded std::ostream insertion operator (operator<<) can be formatted:

class Date {
  int year_, month_, day_;
 public:
  Date(int year, int month, int day) : year_(year), month_(month), day_(day) {}

  friend std::ostream &operator<<(std::ostream &os, const Date &d) {
    return os << d.year_ << '-' << d.month_ << '-' << d.day_;
  }
};

std::string s = fmt::format("The date is {}", Date(2012, 12, 9));
// s == "The date is 2012-12-9"

You can use the FMT_VARIADIC macro to create your own functions similar to format and print which take arbitrary arguments:

// Prints formatted error message.
void report_error(const char *format, const fmt::ArgList &args) {
  fmt::print("Error: ");
  fmt::print(format, args);
}
FMT_VARIADIC(void, report_error, const char *)

report_error("file not found: {}", path);

Note that you only need to define one function that takes const fmt::ArgList & argument and FMT_VARIADIC automatically defines necessary wrappers that accept variable number of arguments. These wrappers are simple inline functions that are very fast and don't result in code bloat.

Projects using this library

• AMPL: Open-source AMPL solver interface, solver connections, table handlers and examples
• Saddy: Small crossplatform 2D graphic engine
• HarpyWar/pvpgn: Player vs Player Gaming Network with tweaks

If you are aware of other projects using this library, please let me know by email or by submitting an issue.

Motivation

So why yet another formatting library?

There are plenty of methods for doing this task, from standard ones like the printf family of function and IOStreams to Boost Format library and FastFormat. The reason for creating a new library is that every existing solution that I found either had serious issues or didn't provide all the features I needed.

Printf

The good thing about printf is that it is very fast and readily available being a part of the C standard library. The main drawback is that it doesn't support user-defined types. Printf also has safety issues although they are mostly solved with __attribute__ ((format (printf, ...)) in GCC. There is a POSIX extension that adds positional arguments required for i18n to printf but it is not a part of C99 and may not be available on some platforms.

IOStreams

The main issue with IOStreams is best illustrated with an example:

std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";

which is a lot of typing compared to printf:

printf("%.2f\n", 1.23456);

Matthew Wilson, the author of FastFormat, referred to this situation with IOStreams as "chevron hell". IOStreams doesn't support positional arguments by design.

The good part is that IOStreams supports user-defined types and is safe although error reporting is awkward.

Boost Format library

This is a very powerful library which supports both printf-like format strings and positional arguments. The main its drawback is performance. According to various benchmarks it is much slower than other methods considered here. Boost Format also has excessive build times and severe code bloat issues (see Benchmarks).

FastFormat

This is an interesting library which is fast, safe and has positional arguments. However it has significant limitations, citing its author:

Three features that have no hope of being accommodated within the current design are:

• Leading zeros (or any other non-space padding)
• Octal/hexadecimal encoding
• Runtime width/alignment specification

It is also quite big and has a heavy dependency, STLSoft, which might be too restrictive for using it in some projects.

Loki SafeFormat

SafeFormat is a formatting library which uses printf-like format strings and is type safe. It doesn't support user-defined types or positional arguments. It makes unconventional use of operator() for passing format arguments.

Tinyformat

This library supports printf-like format strings and is very small and fast. Unfortunately it doesn't support positional arguments and wrapping it in C++98 is somewhat difficult. However if you only need a type-safe printf replacement with support for user-defined types, I highly recommend this library.

Boost Spirit.Karma

This is not really a formatting library but I decided to include it here for completeness. As IOStreams it suffers from the problem of mixing verbatim text with arguments. The library is pretty fast, but slower on integer formatting than fmt::Writer on Karma's own benchmark, see Fast integer to string conversion in C++.

Benchmarks

Speed tests

The following speed tests results were generated by building tinyformat_test.cpp on Ubuntu GNU/Linux 12.10 with g++-4.7.2 -O3 -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 2000000 times with output sent to /dev/null; for further details see the source.

test name	run time
libc printf	1.28s
std::ostream	2.09s
format	1.32s
tinyformat	2.55s
boost::format	10.42s

As you can see boost::format is much slower than the alternative methods; this is confirmed by other tests. Tinyformat is quite good coming close to IOStreams. Unfortunately tinyformat cannot be faster than the IOStreams because it uses them internally. Performance of format is close to that of printf.

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 (g++-4.8.1, Ubuntu GNU/Linux 13.10, best of three) is shown in the following tables.

Optimized build (-O3)

Method	Compile Time, s	Executable size, KiB	Stripped size, KiB
printf	2.6	41	30
IOStreams	19.4	92	70
C++ Format	46.8	46	34
tinyformat	64.6	418	386
Boost Format	222.8	990	923

As you can see, C++ Format has 80% less overhead in terms of resulting code size compared to IOStreams and comes pretty close to printf. Boost Format has by far the largest overheads.

Non-optimized build

Method	Compile Time, s	Executable size, KiB	Stripped size, KiB
printf	2.1	41	30
IOStreams	19.7	86	62
C++ Format	47.9	108	86
tinyformat	27.7	234	190
Boost Format	122.6	884	763

libc, libstdc++ and libformat are all linked as shared libraries to compare formatting function overhead only. Boost Format and tinyformat are header-only libraries so they don't provide any linkage options.

Running the tests

To run the unit tests first get the source code by cloning the repository:

$ git clone https://github.com/cppformat/cppformat.git

or downloading a package from Releases.

Then go to the cppformat directory, generate Makefiles with CMake and build the project:

$ cd cppformat
$ cmake .
$ make

Now you can run the unit tests:

$ make test

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/cppformat/format-benchmark.git
$ cd format-benchmark
$ cmake .

Then you can run the speed test:

$ make speed-test

or the bloat test:

$ make bloat-test

License

Copyright (c) 2012, Victor Zverovich

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Documentation License

The Format String Syntax section in the documentation is based on the one from Python string module documentation adapted for the current library. For this reason the documentation is distributed under the Python Software Foundation license available in doc/LICENSE.python.

Acknowledgments

The benchmark section of this readme file and the performance tests are taken from the excellent tinyformat library written by Chris Foster. Boost Format library is acknowledged transitively since it had some influence on tinyformat. Some ideas used in the implementation are borrowed from Loki SafeFormat and Diagnostic API in Clang. Format string syntax and the documentation are based on Python's str.format. Thanks Doug Turnbull for his valuable comments and contribution to the design of the type-safe API and Gregory Czajkowski for implementing binary formatting. Thanks Ruslan Baratov for comprehensive comparison of integer formatting algorithms and useful comments regarding performance.