363 lines
13 KiB
ReStructuredText
363 lines
13 KiB
ReStructuredText
format
|
|
======
|
|
|
|
Format is an open-source formatting library for C++.
|
|
It can be used as a type-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
|
|
<http://vitaut.github.com/format#format-string-syntax>`__
|
|
similar to the one used by `str.format
|
|
<http://docs.python.org/2/library/stdtypes.html#str.format>`__ in Python.
|
|
* Support for user-defined types.
|
|
* High speed: performance of the format API is close to that of
|
|
glibc's `printf <http://en.cppreference.com/w/cpp/io/c/fprintf>`__
|
|
and better than performance of IOStreams. See `Speed tests`_.
|
|
* 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
|
|
<https://github.com/vitaut/format/blob/master/format_test.cc>`__.
|
|
* 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 <http://vitaut.github.com/format/>`__ for more details.
|
|
|
|
Examples
|
|
--------
|
|
|
|
This prints ``Hello, world!`` to stdout:
|
|
|
|
.. code-block:: c++
|
|
|
|
fmt::Print("Hello, {}!") << "world";
|
|
|
|
Arguments can be accessed by position and arguments' indices can be repeated:
|
|
|
|
.. code-block:: c++
|
|
|
|
std::string s = str(fmt::Format("{0}{1}{0}") << "abra" << "cad");
|
|
// s == "abracadabra"
|
|
|
|
Format can be used as a safe portable replacement for ``itoa``:
|
|
|
|
.. code-block:: c++
|
|
|
|
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
|
|
:code:`std::ostream` insertion operator (``operator<<``) can be formatted:
|
|
|
|
.. code-block:: c++
|
|
|
|
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 = str(fmt::Format("The date is {}") << Date(2012, 12, 9));
|
|
// s == "The date is 2012-12-9"
|
|
|
|
You can use `fmt::Formatter
|
|
<http://zverovich.net/format/#project0classfmt_1_1_formatter>`__
|
|
to create your own functions similar to `fmt::Format
|
|
<http://zverovich.net/format/#fmt::Format__StringRef>`__ and ``fmt::Print``
|
|
with an arbitrary action performed when formatting is complete:
|
|
|
|
.. code-block:: c++
|
|
|
|
struct PrintError {
|
|
void operator()(const fmt::Writer &w) const {
|
|
std::cerr << "Error: " << w.str() << std::endl;
|
|
}
|
|
};
|
|
|
|
// Formats an error message and prints it to std::cerr.
|
|
fmt::Formatter<PrintError> ReportError(const char *format) {
|
|
fmt::Formatter<PrintError> f(format);
|
|
return f;
|
|
}
|
|
|
|
ReportError("File not found: {}") << path;
|
|
|
|
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 the 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, ...))
|
|
<http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`__ in GCC.
|
|
There is a POSIX extension that adds positional arguments required for
|
|
`i18n <http://en.wikipedia.org/wiki/Internationalization_and_localization>`__
|
|
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:
|
|
|
|
.. code-block:: c++
|
|
|
|
std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";
|
|
|
|
which is a lot of typing compared to printf:
|
|
|
|
.. code-block:: c++
|
|
|
|
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, but is faster.
|
|
|
|
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
|
|
<https://github.com/vitaut/tinyformat/blob/master/tinyformat_test.cpp>`__.
|
|
|
|
============== ========
|
|
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 <http://accu.org/index.php/journals/1539>`__.
|
|
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 std::ostream but there is a room for
|
|
improvement since format is not based on IOStreams.
|
|
|
|
Compile time and code bloat
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The script ``bloat_test.sh`` from the `tinyformat
|
|
<https://github.com/c42f/tinyformat>`__ repository 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++-4.7.2, Ubuntu GNU/Linux 12.10, best of three) is shown in the following
|
|
tables.
|
|
|
|
**Non-optimized build**
|
|
|
|
====================== ================== ==========================
|
|
test name total compile time executable size (stripped)
|
|
====================== ================== ==========================
|
|
libc printf 2.8s 44K (32K)
|
|
std::ostream 12.9s 84K (60K)
|
|
format 16.0s 152K (128K)
|
|
tinyformat 20.6s 240K (200K)
|
|
boost::format 76.0s 888K (780K)
|
|
====================== ================== ==========================
|
|
|
|
**Optimized build (-O3)**
|
|
|
|
====================== ================== ==========================
|
|
test name total compile time executable size (stripped)
|
|
====================== ================== ==========================
|
|
libc printf 3.5s 40K (28K)
|
|
std::ostream 14.1s 88K (64K)
|
|
format 25.1s 552K (536K)
|
|
tinyformat 56.3s 200K (164K)
|
|
boost::format 169.4s 1.7M (1.6M)
|
|
====================== ================== ==========================
|
|
|
|
Printf and std::ostream win here which is not surprising considering
|
|
that they are included in the standard library. Tinyformat has somewhat
|
|
slower compilation times compared to format. Interestingly optimized
|
|
executable size is smaller with tinyformat then with format and for
|
|
non-optimized build its the other way around. Boost::format has by far
|
|
the largest overheads.
|
|
|
|
Running the tests
|
|
~~~~~~~~~~~~~~~~~
|
|
|
|
To run the tests you first need to get the format repository with submodules::
|
|
|
|
$ git clone --recursive git://github.com/vitaut/format.git
|
|
|
|
Then go to the format directory and generate Makefiles with
|
|
`CMake <http://www.cmake.org/>`__::
|
|
|
|
$ cd format
|
|
$ cmake .
|
|
|
|
Next use the following commands to run the speed test::
|
|
|
|
$ make speed_test
|
|
|
|
or the bloat test::
|
|
|
|
$ make bloat_test
|
|
|
|
Portability
|
|
-----------
|
|
|
|
The format library has been tested on the following systems and compilers:
|
|
|
|
* 64-bit (amd64) GNU/Linux with GCC 4.6.3 and 4.7.2
|
|
* 32-bit (i386) GNU/Linux with GCC 4.6.3
|
|
* Mac OS X with GCC 4.2.1 and Clang 4.2
|
|
* 64-bit Windows with Visual C++ 2010
|
|
* 32-bit Windows with Visual C++ 2010
|
|
|
|
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
|
|
<http://vitaut.github.com/format#format-string-syntax>`__
|
|
section in the documentation is based on the one from Python `string module
|
|
documentation <http://docs.python.org/3/library/string.html#module-string>`__
|
|
adapted for the current library. For this reason the documentation is
|
|
distributed under the Python Software Foundation license available in
|
|
`doc/LICENSE.python
|
|
<https://raw.github.com/vitaut/format/master/doc/LICENSE.python>`__.
|
|
|
|
Acknowledgments
|
|
---------------
|
|
|
|
The benchmark section of this readme file and the performance tests are taken
|
|
from the excellent `tinyformat <https://github.com/c42f/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
|
|
<http://loki-lib.sourceforge.net/>`__ SafeFormat and `Diagnostic API
|
|
<http://clang.llvm.org/doxygen/classclang_1_1Diagnostic.html>`__ in
|
|
`Clang <http://clang.llvm.org/>`__.
|
|
Format string syntax and the documentation are based on Python's `str.format
|
|
<http://docs.python.org/2/library/stdtypes.html#str.format>`__.
|
|
Thanks `Doug Turnbull <https://github.com/softwaredoug>`__ for his valuable
|
|
comments and contribution to the design of the type-safe API and
|
|
`Gregory Czajkowski <https://github.com/gcflymoto>`__ for implementing binary
|
|
formatting.
|