fmt/doc/index.md

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# A modern formatting library

<div class="features-container">

<div class="feature">
<h2>Safety</h2>
<p>
  Inspired by 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:

  <pre><code class="language-cpp"
  >fmt::format("{:d}", "I am not a number");</code></pre>

  will give a compile-time error because <code>d</code> is not a valid
  format specifier for strings. APIs like <a href="api/#format">
  <code>fmt::format</code></a> <b>prevent buffer overflow errors</b> via
  automatic memory management.
</p>
<a href="api#compile-time-checks">→ Learn more</a>
</div>

<div class="feature">
<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>

  prints the vector in a JSON-like format:

  <pre><code>[1, 2, 3]</code></pre>

  You can <b>make your own types formattable</b> and even make compile-time
  checks work for them.
</p>
<a href="api#udt">→ Learn more</a>
</div>

<div class="feature">
<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 for numeric formatting.

<a href="https://github.com/fmtlib/fmt?tab=readme-ov-file#benchmarks">
<img src="perf.svg">
</a>

  The library <b>minimizes dynamic memory allocations</b> and can optionally
  <a href="api#compile-api">compile format strings</a> to optimal code.
</p>
</div>

<div class="feature">
<h2>Unicode support</h2>
<p>
  {fmt} provides <b>portable Unicode support</b> on major operating systems
  with UTF-8 and <code>char</code> strings. For example:

  <pre><code class="language-cpp"
  >fmt::print("Слава Україні!");</code></pre>

  will be printed correctly on Linux, macOS, and even Windows console,
  irrespective 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, addressing issues in the
  standard libary.
</p>
</div>

<div class="feature">
<h2>Fast compilation</h2>
<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> compiles 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>

<div class="feature">
<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 just <a href="https://godbolt.org/g/TZU4KF">a few
  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>

<div class="feature">
<h2>Portability</h2>
<p>
  {fmt} has a <b>small self-contained codebase</b> with the core consisting of
  just three headers and no external dependencies.
</p>
<p>
  The library is highly portable and requires only a minimal <b>subset of
  C++11</b> features which are available in GCC 4.9, 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>

<div class="feature">
<h2>Open source</h2>
<p>
  {fmt} is in the top hundred open-source C++ libraries on GitHub and has
  <a href="https://github.com/fmtlib/fmt/graphs/contributors">hundreds of
  all-time contributors</a>.
</p>
<p>
  The library is distributed under a permissive MIT
  <a href="https://github.com/fmtlib/fmt#license">license</a> and is
  <b>relied upon by many open-source projects</b>, including Blender, PyTorch,
  Apple's FoundationDB, Windows Terminal, MongoDB, and others.
</p>
</div>

</div>