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# Overview ---
hide:
- navigation
- toc
---
**{fmt}** is an open-source formatting library providing a fast and safe # A modern formatting library
alternative to C stdio and C++ iostreams.
What users say: <div class="features-container">
> Thanks for creating this library. It’s been a hole in C++ for a long time. <div class="feature">
> I’ve used both `boost::format` and `loki::SPrintf`, and neither felt like the <h2>Safety</h2>
> right answer. This does. <p>
Inspired by the Python's formatting facility, {fmt} provides a safe
replacement for the <code>printf</code> family of functions. Errors in format
strings, which are a common source of vulnerabilities in C, are <b>reported
at compile time</b>. For example:
## Format API <pre><code class="language-cpp"
>fmt::format("The answer is {:d}", "forty-two");</code></pre>
The format API is similar in spirit to the C `printf` family of function will give a compile-time error because <code>d</code> is not a valid
but is safer, simpler and several times format specifier for strings. APIs like <a href="api/#format">
[faster](https://vitaut.net/posts/2020/fast-int-to-string-revisited/) <code>fmt::format</code></a> <b>prevent buffer overflow errors</b> via
than common standard library implementations. automatic memory management.
The [format string syntax](syntax.md) is similar to the one used by </p>
[str.format](https://docs.python.org/3/library/stdtypes.html#str.format) <a href="api#compile-time-format-string-checks">→ Learn more</a>
in Python: </div>
```c++ <div class="feature">
std::string s = fmt::format("The answer is {}.", 42); <h2>Extensibility</h2>
``` <p>
Formatting of most <b>standard types</b> including all containers, dates and
times is <b>supported out-of-the-box</b>.
For example:
<pre><code class="language-cpp"
>fmt::print("{}", std::vector{1, 2, 3});</code></pre>
The `fmt::format` function returns a string \"The answer is 42.\". You prints the vector in a JSON-like format:
can use `fmt::memory_buffer` to avoid constructing `std::string`:
```c++ <pre><code>[1, 2, 3]</code></pre>
auto out = fmt::memory_buffer();
fmt::format_to(std::back_inserter(out),
"For a moment, {} happened.", "nothing");
auto data = out.data(); // pointer to the formatted data
auto size = out.size(); // size of the formatted data
```
The `fmt::print` function performs formatting and writes the result to a You can <b>make your own types formattable</b> and even make compile-time
stream: checks work for them.
</p>
<a href="api#udt">→ Learn more</a>
</div>
```c++ <div class="feature">
fmt::print(stderr, "System error code = {}\n", errno); <h2>Performance</h2>
``` <p>
{fmt} can be anywhere from <b>tens of percent to 20-30 times faster</b> than
iostreams and <code>sprintf</code>, especially on numeric formatting.
If you omit the file argument the function will print to `stdout`: <svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewBox="20 0 550 300" aria-label="A chart." style="overflow: hidden;"><defs id="_ABSTRACT_RENDERER_ID_0"><clipPath id="_ABSTRACT_RENDERER_ID_1"><rect x="120" y="45" width="560" height="210"></rect></clipPath></defs><rect x="0" y="0" width="800" height="300" stroke="none" stroke-width="0" fill="#ffffff"></rect><g><text text-anchor="start" x="120" y="27.05" font-family="Arial" font-size="13" font-weight="bold" stroke="none" stroke-width="0" fill="#000000">double to string</text><rect x="120" y="16" width="560" height="13" stroke="none" stroke-width="0" fill-opacity="0" fill="#ffffff"></rect></g><g><rect x="120" y="45" width="560" height="210" stroke="none" stroke-width="0" fill-opacity="0" fill="#ffffff"></rect><g clip-path="url(#_ABSTRACT_RENDERER_ID_1)"><g><rect x="120" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#cccccc"></rect><rect x="213" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#cccccc"></rect><rect x="306" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#cccccc"></rect><rect x="400" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#cccccc"></rect><rect x="493" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#cccccc"></rect><rect x="586" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#cccccc"></rect><rect x="679" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#cccccc"></rect></g><g><rect x="121" y="53" width="450" height="26" stroke="#ff9900" stroke-width="1" fill="#ff9900"></rect><rect x="121" y="95" width="421" height="26" stroke="#109618" stroke-width="1" fill="#109618"></rect><rect x="121" y="137" width="341" height="26" stroke="#990099" stroke-width="1" fill="#990099"></rect><rect x="121" y="179" width="31" height="26" stroke="#3366cc" stroke-width="1" fill="#3366cc"></rect><rect x="121" y="221" width="15" height="26" stroke="#dc3912" stroke-width="1" fill="#dc3912"></rect></g><g><rect x="120" y="45" width="1" height="210" stroke="none" stroke-width="0" fill="#333333"></rect></g></g><g></g><g><g><text text-anchor="middle" x="120.5" y="272.3833333333333" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#444444">0</text></g><g><text text-anchor="middle" x="213.6667" y="272.3833333333333" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#444444">250</text></g><g><text text-anchor="middle" x="306.8333" y="272.3833333333333" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#444444">500</text></g><g><text text-anchor="middle" x="400" y="272.3833333333333" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#444444">750</text></g><g><text text-anchor="middle" x="493.1667" y="272.3833333333333" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#444444">1,000</text></g><g><text text-anchor="middle" x="586.3333" y="272.3833333333333" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#444444">1,250</text></g><g><text text-anchor="middle" x="679.5" y="272.3833333333333" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#444444">1,500</text></g><g><text text-anchor="end" x="107" y="70.95" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#222222">ostringstream</text></g><g><text text-anchor="end" x="107" y="112.74999999999999" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#222222">ostrstream</text></g><g><text text-anchor="end" x="107" y="154.55" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#222222">sprintf</text></g><g><text text-anchor="end" x="107" y="196.35" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#222222">doubleconv</text></g><g><text text-anchor="end" x="107" y="238.15" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#222222">fmt</text></g></g></g><g><g><text text-anchor="middle" x="300" y="291.71666666666664" font-family="Arial" font-size="13" font-style="italic" stroke="none" stroke-width="0" fill="#222222">Time (ns), smaller is better</text><rect x="120" y="280.66666666666663" width="560" height="13" stroke="none" stroke-width="0" fill-opacity="0" fill="#ffffff"></rect></g></g><g></g></svg>
```c++ The library <b>minimizes dynamic memory allocations</b> and allows
fmt::print("Don't {}\n", "panic"); <a href="api#compile-api">format string compilation</a>.
``` </p>
</div>
The format API also supports positional arguments useful for <div class="feature">
localization: <h2>Unicode support</h2>
<p>
{fmt} provides <b>portable Unicode support</b> on major operating systems
with UTF-8 and normal <code>char</code> strings. For example:
```c++ <pre><code class="language-cpp"
fmt::print("I'd rather be {1} than {0}.", "right", "happy"); >fmt::print("Слава Україні!");</code></pre>
```
You can pass named arguments with `fmt::arg`: will be printed correctly on Linux, macOS and even Windows console regardless
of the codepages.
</p>
<p>
The default is <b>locale-independent</b> but you can opt into localized
formatting and {fmt} makes it work with Unicode, working around problems in
the standard libary.
</p>
</div>
```c++ <div class="feature">
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.", <h2>Fast compilation</h2>
fmt::arg("name", "World"), fmt::arg("number", 42)); <p>
``` The library makes extensive use of <b>type erasure</b> to achieve fast
compilation. <code>fmt/base.h</code> provides a subset
of the API with <b>minimal include dependencies</b> and enough functionality
to replace all uses of <code>*printf</code>.
</p>
<p>
Code using {fmt} is usually several times faster to compile than the
equivalent iostreams code and while <code>printf</code> is faster still, the
gap is narrowing.
</p>
<a href="https://github.com/fmtlib/fmt?tab=readme-ov-file#compile-time-and-code-bloat">
→ Learn more</a>
</div>
If your compiler supports C++11 user-defined literals, the suffix `_a` <div class="feature">
offers an alternative, slightly terser syntax for named arguments: <h2>Small binary footprint</h2>
<p>
Type erasure is also used to prevent template bloat resulting in <b>compact
per-call binary code</b>. For example, a call to <code>fmt::print</code> with
a single argument is less than <a href="https://godbolt.org/g/TZU4KF">ten
x86-64 instructions</a>, comparable to <code>printf</code> despite adding
runtime safety and much smaller than the equivalent iostreams code.
</p>
<p>
The library itself has small binary footprint and some components such as
floating-point formatting can be disabled to make it even smaller for
resource constrained devices.
</p>
</div>
```c++ <div class="feature">
using namespace fmt::literals; <h2>Portability</h2>
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.", <p>
"name"_a="World", "number"_a=42); {fmt} has a <b>small self-contained codebase</b> with the core consisting of
``` just three header files and no external dependencies.
</p>
<p>
The library is highly portable and requires only on a minimal <b>subset of
C++11</b> features which are available in GCC 4.8, Clang 3.4, MSVC 19.0 (2015)
and later. Newer compiler and standard library features are used if available
and enable additional functionality.
</p>
<p>
Where possible, the output of formatting functions is <b>consistent across
platforms</b>.
</p>
</p>
</div>
## Safety <div class="feature">
<h2>Open source</h2>
<p>
{fmt} is in top hundred open-source libraries on GitHub and has <b>hundreds of
all-time contributors</b>.
</p>
<p>
Permissive MIT <a href="https://github.com/fmtlib/fmt#license">license</a>
allows using the library both in open-source and commercial projects.
</p>
</div>
The library is fully type safe, automatic memory management prevents </div>
buffer overflow, errors in format strings are reported using exceptions
or at compile time. For example, the 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
```c++
format(FMT_STRING("The answer is {:d}"), "forty-two");
```
reports a compile-time error on compilers that support relaxed `constexpr`.
See [Compile-Time Format String Checks](api.md#compile-time-format-string-checks)
for details.
The following 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. 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)),
```c++
#include <fmt/core.h>
int main() {
fmt::print("The answer is {}.", 42);
}
```
compiles to just
```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
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 3.4, 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,
```
fmt::print("{}", std::numeric_limits<double>::infinity());
```
always prints `inf` while the output of `printf` is platform-dependent.
## 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.
<a class="btn btn-success" href="https://github.com/fmtlib/fmt">
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