|
|
Overview
|
|
|
========
|
|
|
|
|
|
**{fmt}** is an open-source formatting library providing a fast and safe
|
|
|
alternative to C stdio and 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 <code>boost::format</code> and
|
|
|
<code>loki::SPrintf</code>, and neither felt like the right answer.
|
|
|
This does.
|
|
|
</div>
|
|
|
</div>
|
|
|
|
|
|
.. _format-api-intro:
|
|
|
|
|
|
Format API
|
|
|
----------
|
|
|
|
|
|
The format API is similar in spirit to the C ``printf`` family of function but
|
|
|
is safer, simpler and several times `faster
|
|
|
<https://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html>`_
|
|
|
than common standard library implementations.
|
|
|
The `format string syntax <syntax.html>`_ is similar to the one used by
|
|
|
`str.format <http://docs.python.org/3/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::memory_buffer`` to avoid constructing ``std::string``:
|
|
|
|
|
|
.. code:: c++
|
|
|
|
|
|
fmt::memory_buffer out;
|
|
|
format_to(out, "For a moment, {} happened.", "nothing");
|
|
|
out.data(); // returns a pointer to the formatted data
|
|
|
|
|
|
The ``fmt::print`` function performs formatting and writes the result to a stream:
|
|
|
|
|
|
.. code:: c++
|
|
|
|
|
|
fmt::print(stderr, "System error code = {}\n", errno);
|
|
|
|
|
|
If you omit the file argument the function will print to ``stdout``:
|
|
|
|
|
|
.. code:: c++
|
|
|
|
|
|
fmt::print("Don't {}\n", "panic");
|
|
|
|
|
|
The format API also supports positional arguments useful for localization:
|
|
|
|
|
|
.. code:: c++
|
|
|
|
|
|
fmt::print("I'd rather be {1} than {0}.", "right", "happy");
|
|
|
|
|
|
You can pass named arguments with ``fmt::arg``:
|
|
|
|
|
|
.. 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++
|
|
|
|
|
|
using namespace fmt::literals;
|
|
|
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
|
|
|
"name"_a="World", "number"_a=42);
|
|
|
|
|
|
.. _safety:
|
|
|
|
|
|
Safety
|
|
|
------
|
|
|
|
|
|
The library is fully type safe, automatic memory management prevents buffer
|
|
|
overflow, errors in format strings are reported using exceptions or at compile
|
|
|
time. For example, the code
|
|
|
|
|
|
.. code:: c++
|
|
|
|
|
|
fmt::format("The answer is {:d}", "forty-two");
|
|
|
|
|
|
throws the ``format_error`` exception because the argument ``"forty-two"`` is a
|
|
|
string while the format code ``d`` only applies to integers.
|
|
|
|
|
|
The code
|
|
|
|
|
|
.. code:: c++
|
|
|
|
|
|
format(FMT_STRING("The answer is {:d}"), "forty-two");
|
|
|
|
|
|
reports a compile-time error on compilers that support relaxed ``constexpr``.
|
|
|
See `here <api.html#c.fmt>`_ for details.
|
|
|
|
|
|
The following 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
|
|
|
desirable.
|
|
|
|
|
|
Compact Binary Code
|
|
|
-------------------
|
|
|
|
|
|
The library produces compact per-call compiled code. For example
|
|
|
(`godbolt <https://godbolt.org/g/TZU4KF>`_),
|
|
|
|
|
|
.. code:: c++
|
|
|
|
|
|
#include <fmt/core.h>
|
|
|
|
|
|
int main() {
|
|
|
fmt::print("The answer is {}.", 42);
|
|
|
}
|
|
|
|
|
|
compiles to just
|
|
|
|
|
|
.. code:: asm
|
|
|
|
|
|
main: # @main
|
|
|
sub rsp, 24
|
|
|
mov qword ptr [rsp], 42
|
|
|
mov rcx, rsp
|
|
|
mov edi, offset .L.str
|
|
|
mov esi, 17
|
|
|
mov edx, 1
|
|
|
call fmt::v7::vprint(fmt::v7::basic_string_view<char>, fmt::v7::format_args)
|
|
|
xor eax, eax
|
|
|
add rsp, 24
|
|
|
ret
|
|
|
.L.str:
|
|
|
.asciz "The answer is {}."
|
|
|
|
|
|
.. _portability:
|
|
|
|
|
|
Portability
|
|
|
-----------
|
|
|
|
|
|
The library is highly portable and relies only on a small set of C++11 features:
|
|
|
|
|
|
* variadic templates
|
|
|
* type traits
|
|
|
* rvalue references
|
|
|
* decltype
|
|
|
* trailing return types
|
|
|
* deleted functions
|
|
|
* alias templates
|
|
|
|
|
|
These are available in GCC 4.8, Clang 3.0, MSVC 19.0 (2015) and more recent
|
|
|
compiler version. For older compilers use {fmt} `version 4.x
|
|
|
<https://github.com/fmtlib/fmt/releases/tag/4.1.0>`_ which is maintained and
|
|
|
only requires C++98.
|
|
|
|
|
|
The output of all formatting functions is consistent across platforms.
|
|
|
For example,
|
|
|
|
|
|
.. code::
|
|
|
|
|
|
fmt::print("{}", std::numeric_limits<double>::infinity());
|
|
|
|
|
|
always prints ``inf`` while the output of ``printf`` is platform-dependent.
|
|
|
|
|
|
.. _ease-of-use:
|
|
|
|
|
|
Ease of Use
|
|
|
-----------
|
|
|
|
|
|
{fmt} has a small self-contained code base with the core library consisting of
|
|
|
just three header files and no external dependencies.
|
|
|
A permissive MIT `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>
|
