206 lines
6.4 KiB
ReStructuredText
206 lines
6.4 KiB
ReStructuredText
Overview
|
||
========
|
||
|
||
**fmt** (formerly cppformat) is an open-source formatting library.
|
||
It can be used as a safe alternative to printf or as a fast
|
||
alternative to C++ IOStreams.
|
||
|
||
.. raw:: html
|
||
|
||
<div class="panel panel-default">
|
||
<div class="panel-heading">What users say:</div>
|
||
<div class="panel-body">
|
||
Thanks for creating this library. It’s been a hole in C++ for a long
|
||
time. I’ve used both boost::format and loki::SPrintf, and neither felt
|
||
like the right answer. This does.
|
||
</div>
|
||
</div>
|
||
|
||
.. _format-api:
|
||
|
||
Format API
|
||
----------
|
||
|
||
The replacement-based Format API provides a safe alternative to ``printf``,
|
||
``sprintf`` and friends with comparable or `better performance
|
||
<http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html>`_.
|
||
The `format string syntax <syntax.html>`_ is similar to the one used by
|
||
`str.format <http://docs.python.org/2/library/stdtypes.html#str.format>`_
|
||
in Python:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::format("The answer is {}", 42);
|
||
|
||
The ``fmt::format`` function returns a string "The answer is 42". You can use
|
||
``fmt::MemoryWriter`` to avoid constructing ``std::string``:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::MemoryWriter w;
|
||
w.write("Look, a {} string", 'C');
|
||
w.c_str(); // returns a C string (const char*)
|
||
|
||
The ``fmt::print`` function performs formatting and writes the result to a file:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::print(stderr, "System error code = {}\n", errno);
|
||
|
||
The file argument can be omitted in which case the function prints to
|
||
``stdout``:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::print("Don't {}\n", "panic");
|
||
|
||
If your compiler supports C++11, then the formatting functions are implemented
|
||
with variadic templates. Otherwise variadic functions are emulated by generating
|
||
a set of lightweight wrappers. This ensures compatibility with older compilers
|
||
while providing a natural API.
|
||
|
||
The Format API also supports positional arguments useful for localization:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::print("I'd rather be {1} than {0}.", "right", "happy");
|
||
|
||
Named arguments can be created with ``fmt::arg``. This makes it easier to track
|
||
what goes where when multiple values are being inserted:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
|
||
fmt::arg("name", "World"), fmt::arg("number", 42));
|
||
|
||
If your compiler supports C++11 user-defined literals, the suffix ``_a`` offers
|
||
an alternative, slightly terser syntax for named arguments:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
|
||
"name"_a="World", "number"_a=42);
|
||
|
||
The ``_format`` suffix may be used to format string literals similar to Python:
|
||
|
||
.. code:: c++
|
||
|
||
std::string message = "{0}{1}{0}"_format("abra", "cad");
|
||
|
||
Other than the placement of the format string on the left of the operator,
|
||
``_format`` is functionally identical to ``fmt::format``. In order to use the
|
||
literal operators, they must be made visible with the directive
|
||
``using namespace fmt::literals;``. Note that this brings in only ``_a`` and
|
||
``_format`` but nothing else from the ``fmt`` namespace.
|
||
|
||
.. _write-api:
|
||
|
||
Write API
|
||
---------
|
||
|
||
The concatenation-based Write API (experimental) provides a `fast
|
||
<http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html>`_
|
||
stateless alternative to IOStreams:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::MemoryWriter out;
|
||
out << "The answer in hexadecimal is " << hex(42);
|
||
|
||
.. _safety:
|
||
|
||
Safety
|
||
------
|
||
|
||
The library is fully type safe, automatic memory management prevents buffer
|
||
overflow, errors in format strings are reported using exceptions. For example,
|
||
the code
|
||
|
||
.. code:: c++
|
||
|
||
fmt::format("The answer is {:d}", "forty-two");
|
||
|
||
throws a ``FormatError`` exception with description
|
||
"unknown format code 'd' for string", because the argument
|
||
``"forty-two"`` is a string while the format code ``d``
|
||
only applies to integers.
|
||
|
||
Where possible, errors are caught at compile time. For example, the code
|
||
|
||
.. code:: c++
|
||
|
||
fmt::format("Cyrillic letter {}", L'\x42e');
|
||
|
||
produces a compile-time error because wide character ``L'\x42e'`` cannot be
|
||
formatted into a narrow string. You can use a wide format string instead:
|
||
|
||
.. code:: c++
|
||
|
||
fmt::format(L"Cyrillic letter {}", L'\x42e');
|
||
|
||
For comparison, writing a wide character to ``std::ostream`` results in
|
||
its numeric value being written to the stream (i.e. 1070 instead of letter 'ю'
|
||
which is represented by ``L'\x42e'`` if we use Unicode) which is rarely what is
|
||
needed.
|
||
|
||
.. _portability:
|
||
|
||
Portability
|
||
-----------
|
||
|
||
The library is highly portable. Here is an incomplete list of operating systems
|
||
and compilers where it has been tested and known to work:
|
||
|
||
* 64-bit (amd64) GNU/Linux with GCC 4.4.3,
|
||
`4.6.3 <https://travis-ci.org/fmtlib/fmt>`_, 4.7.2, 4.8.1, and Intel C++
|
||
Compiler (ICC) 14.0.2
|
||
|
||
* 32-bit (i386) GNU/Linux with GCC 4.4.3, 4.6.3
|
||
|
||
* Mac OS X with GCC 4.2.1 and Clang 4.2, 5.1.0
|
||
|
||
* 64-bit Windows with Visual C++ 2010, 2013 and
|
||
`2015 <https://ci.appveyor.com/project/vitaut/fmt>`_
|
||
|
||
* 32-bit Windows with Visual C++ 2010
|
||
|
||
Although the library uses C++11 features when available, it also works with
|
||
older compilers and standard library implementations. The only thing to keep in
|
||
mind for C++98 portability:
|
||
|
||
* Variadic templates: minimum GCC 4.4, Clang 2.9 or VS2013. This feature allows
|
||
the Format API to accept an unlimited number of arguments. With older
|
||
compilers the maximum is 15.
|
||
|
||
* User-defined literals: minimum GCC 4.7, Clang 3.1 or VS2015. The suffixes
|
||
``_format`` and ``_a`` are functionally equivalent to the functions
|
||
``fmt::format`` and ``fmt::arg``.
|
||
|
||
The output of all formatting functions is consistent across platforms. In
|
||
particular, formatting a floating-point infinity always gives ``inf`` while the
|
||
output of ``printf`` is platform-dependent in this case. For example,
|
||
|
||
.. code::
|
||
|
||
fmt::print("{}", std::numeric_limits<double>::infinity());
|
||
|
||
always prints ``inf``.
|
||
|
||
.. _ease-of-use:
|
||
|
||
Ease of Use
|
||
-----------
|
||
|
||
fmt has a small self-contained code base with the core library consisting of
|
||
a single header file and a single source file and no external dependencies.
|
||
A permissive BSD `license <https://github.com/fmtlib/fmt#license>`_ allows
|
||
using the library both in open-source and commercial projects.
|
||
|
||
.. raw:: html
|
||
|
||
<a class="btn btn-success" href="https://github.com/fmtlib/fmt">GitHub Repository</a>
|
||
|
||
<div class="section footer">
|
||
<iframe src="http://ghbtns.com/github-btn.html?user=fmtlib&repo=fmt&type=watch&count=true"
|
||
class="github-btn" width="100" height="20"></iframe>
|
||
</div>
|