API - {fmt}

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</div></a> </div> <ul class="md-nav__list" data-md-scrollfix> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB" class="md-nav__link"> <span class="md-ellipsis"> Home </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/get-started/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB" class="md-nav__link"> <span class="md-ellipsis"> Get Started </span> </a></li> <li class="md-nav__item md-nav__item--active"><input class="md-nav__toggle md-toggle" type="checkbox" id="__toc"> <label class="md-nav__link md-nav__link--active" for="__toc"> <span class="md-ellipsis"> API </span> <span class="md-nav__icon md-icon"></span> </label> <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB" class="md-nav__link md-nav__link--active"> <span class="md-ellipsis"> API </span> </a> <nav class="md-nav md-nav--secondary" aria-label="Table of contents"><label class="md-nav__title" for="__toc"> <span class="md-nav__icon md-icon"></span> Table of contents </label> <ul class="md-nav__list" data-md-component="toc" data-md-scrollfix> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#base-api" class="md-nav__link"> <span class="md-ellipsis"> Base API </span> </a> <nav class="md-nav" aria-label="Base API"> <ul class="md-nav__list"> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#formatting-user-defined-types" class="md-nav__link"> <span class="md-ellipsis"> Formatting User-Defined Types </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#compile-time-checks" class="md-nav__link"> <span class="md-ellipsis"> Compile-Time Checks </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#named-arguments" class="md-nav__link"> <span class="md-ellipsis"> Named Arguments </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#type-erasure" class="md-nav__link"> <span class="md-ellipsis"> Type Erasure </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#compatibility" class="md-nav__link"> <span class="md-ellipsis"> Compatibility </span> </a></li> </ul> </nav></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#format-api" class="md-nav__link"> <span class="md-ellipsis"> Format API </span> </a> <nav class="md-nav" aria-label="Format API"> <ul class="md-nav__list"> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#utilities" class="md-nav__link"> <span class="md-ellipsis"> Utilities </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#system-errors" class="md-nav__link"> <span class="md-ellipsis"> System Errors </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#custom-allocators" class="md-nav__link"> <span class="md-ellipsis"> Custom Allocators </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#locale" class="md-nav__link"> <span class="md-ellipsis"> Locale </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#legacy-compile-time-checks" class="md-nav__link"> <span class="md-ellipsis"> Legacy Compile-Time Checks </span> </a></li> </ul> </nav></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#range-and-tuple-formatting" class="md-nav__link"> <span class="md-ellipsis"> Range and Tuple Formatting </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#date-and-time-formatting" class="md-nav__link"> <span class="md-ellipsis"> Date and Time Formatting </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#standard-library-types-formatting" class="md-nav__link"> <span class="md-ellipsis"> Standard Library Types Formatting </span> </a> <nav class="md-nav" aria-label="Standard Library Types Formatting"> <ul class="md-nav__list"> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#variants" class="md-nav__link"> <span class="md-ellipsis"> Variants </span> </a></li> </ul> </nav></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#bit-fields-and-packed-structs" class="md-nav__link"> <span class="md-ellipsis"> Bit-Fields and Packed Structs </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#format-string-compilation" class="md-nav__link"> <span class="md-ellipsis"> Format String Compilation </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#terminal-colors-and-text-styles" class="md-nav__link"> <span class="md-ellipsis"> Terminal Colors and Text Styles </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#system-apis" class="md-nav__link"> <span class="md-ellipsis"> System APIs </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#stdostream-support" class="md-nav__link"> <span class="md-ellipsis"> std::ostream Support </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#dynamic-argument-lists" class="md-nav__link"> <span class="md-ellipsis"> Dynamic Argument Lists </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#safe-printf" class="md-nav__link"> <span class="md-ellipsis"> Safe printf </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#wide-strings" class="md-nav__link"> <span class="md-ellipsis"> Wide Strings </span> </a></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#compatibility-with-c20-stdformat" class="md-nav__link"> <span class="md-ellipsis"> Compatibility with C++20 std::format </span> </a></li> </ul> </nav></li> <li class="md-nav__item"><a href="https://fmt-dev.translate.goog/latest/syntax/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB" class="md-nav__link"> <span class="md-ellipsis"> Syntax </span> </a></li> </ul> </nav> </div> </div> </div> <div class="md-content" data-md-component="content"> <article class="md-content__inner md-typeset"> <h1 id="api-reference">API Reference</h1> <p>The {fmt} library API consists of the following components:</p> <ul> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#base-api"><code>fmt/base.h</code></a>: the base API providing main formatting functions for <code>char</code>/UTF-8 with C++20 compile-time checks and minimal dependencies</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#format-api"><code>fmt/format.h</code></a>: <code>fmt::format</code> and other formatting functions as well as locale support</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#ranges-api"><code>fmt/ranges.h</code></a>: formatting of ranges and tuples</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#chrono-api"><code>fmt/chrono.h</code></a>: date and time formatting</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#std-api"><code>fmt/std.h</code></a>: formatters for standard library types</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#compile-api"><code>fmt/compile.h</code></a>: format string compilation</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#color-api"><code>fmt/color.h</code></a>: terminal colors and text styles</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#os-api"><code>fmt/os.h</code></a>: system APIs</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#ostream-api"><code>fmt/ostream.h</code></a>: <code>std::ostream</code> support</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#args-api"><code>fmt/args.h</code></a>: dynamic argument lists</li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#printf-api"><code>fmt/printf.h</code></a>: safe <code>printf</code></li> <li><a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#xchar-api"><code>fmt/xchar.h</code></a>: optional <code>wchar_t</code> support</li> </ul> <p>All functions and types provided by the library reside in namespace <code>fmt</code> and macros have prefix <code>FMT_</code>.</p> <h2 id="base-api">Base API</h2> <p><code>fmt/base.h</code> defines the base API which provides main formatting functions for <code>char</code>/UTF-8 with C++20 compile-time checks. It has minimal include dependencies for better compile times. This header is only beneficial when using {fmt} as a library (the default) and not in the header-only mode. It also provides <code>formatter</code> specializations for the following types:</p> <ul> <li><code>int</code>, <code>long long</code>,</li> <li><code>unsigned</code>, <code>unsigned long long</code></li> <li><code>float</code>, <code>double</code>, <code>long double</code></li> <li><code>bool</code></li> <li><code>char</code></li> <li><code>const char*</code>, <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#basic_string_view"><code>fmt::string_view</code></a></li> <li><code>const void*</code></li> </ul> <p>The following functions use <a href="https://fmt-dev.translate.goog/latest/syntax/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB">format string syntax</a> similar to that of <a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://docs.python.org/3/library/stdtypes.html%23str.format">str.format</a> in Python. They take <em>fmt</em> and <em>args</em> as arguments.</p> <p><em>fmt</em> is a format string that contains literal text and replacement fields surrounded by braces <code>{}</code>. The fields are replaced with formatted arguments in the resulting string. <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#format_string"><code>fmt::format_string</code></a> is a format string which can be implicitly constructed from a string literal or a <code>constexpr</code> string and is checked at compile time in C++20. To pass a runtime format string wrap it in <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#runtime"><code>fmt::runtime</code></a>.</p> <p><em>args</em> is an argument list representing objects to be formatted.</p> <p>I/O errors are reported as <a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/error/system_error"><code>std::system_error</code></a> exceptions unless specified otherwise.</p> <div class="docblock"><a id="print"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>void print(format_string<T...> fmt, T&&... args); </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and writes the output to <code>stdout</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print("The answer is {}.", 42); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="print"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>void print(FILE* f, format_string<T...> fmt, T&&... args); </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and writes the output to the file <code>f</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print(stderr, "Don't {}!", "panic"); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="println"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>void println(format_string<T...> fmt, T&&... args); </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and writes the output to <code>stdout</code> followed by a newline.</p> </div> </div> <div class="docblock"><a id="println"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>void println(FILE* f, format_string<T...> fmt, T&&... args); </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and writes the output to the file <code>f</code> followed by a newline.</p> </div> </div> <div class="docblock"><a id="format_to"> <pre><code class="language-cpp decl"> <div>template <typename OutputIt, typename... T> </div> <div>auto format_to(OutputIt&& out, format_string<T...> fmt, T&&... args) -⁠> remove_cvref_t<OutputIt>; </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code>, writes the result to the output iterator <code>out</code> and returns the iterator past the end of the output range. <code>format_to</code> does not append a terminating null character.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">auto out = std::vector<char>(); fmt::format_to(std::back_inserter(out), "{}", 42); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="format_to_n"> <pre><code class="language-cpp decl"> <div>template <typename OutputIt, typename... T> </div> <div>auto format_to_n(OutputIt out, size_t n, format_string<T...> fmt, T&&... args) -⁠> format_to_n_result<OutputIt>; </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code>, writes up to <code>n</code> characters of the result to the output iterator <code>out</code> and returns the total (not truncated) output size and the iterator past the end of the output range. <code>format_to_n</code> does not append a terminating null character.</p> </div> </div> <div class="docblock"><a id="format_to_n_result"> <pre><code class="language-cpp decl"> <div>template <typename OutputIt> </div> <div>struct format_to_n_result; </div></code></pre></a> <div class="docblock-desc"> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>OutputIt out; </div></code></pre> <div class="docblock-desc"> <p>Iterator past the end of the output range.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>size_t size; </div></code></pre> <div class="docblock-desc"> <p>Total (not truncated) output size.</p> </div> </div> </div> </div> <div class="docblock"><a id="formatted_size"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>auto formatted_size(format_string<T...> fmt, T&&... args) -⁠> size_t; </div></code></pre></a> <div class="docblock-desc"> <p>Returns the number of chars in the output of <code>format(fmt, args...)</code>.</p> </div> </div> <p><a id="udt"></a></p> <h3 id="formatting-user-defined-types">Formatting User-Defined Types</h3> <p>The {fmt} library provides formatters for many standard C++ types. See <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#ranges-api"><code>fmt/ranges.h</code></a> for ranges and tuples including standard containers such as <code>std::vector</code>, <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#chrono-api"><code>fmt/chrono.h</code></a> for date and time formatting and <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#std-api"><code>fmt/std.h</code></a> for other standard library types.</p> <p>There are two ways to make a user-defined type formattable: providing a <code>format_as</code> function or specializing the <code>formatter</code> struct template.</p> <p>Use <code>format_as</code> if you want to make your type formattable as some other type with the same format specifiers. The <code>format_as</code> function should take an object of your type and return an object of a formattable type. It should be defined in the same namespace as your type.</p> <p>Example (<a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://godbolt.org/z/nvME4arz8">run</a>):</p> <pre class="highlight"><code>#include <fmt/format.h> namespace kevin_namespacy { enum class film { house_of_cards, american_beauty, se7en = 7 }; auto format_as(film f) { return fmt::underlying(f); } } int main() { fmt::print("{}\n", kevin_namespacy::film::se7en); // Output: 7 }</code></pre> <p>Using specialization is more complex but gives you full control over parsing and formatting. To use this method specialize the <code>formatter</code> struct template for your type and implement <code>parse</code> and <code>format</code> methods.</p> <p>The recommended way of defining a formatter is by reusing an existing one via inheritance or composition. This way you can support standard format specifiers without implementing them yourself. For example:</p> <pre><code class="language-c++">// color.h: #include <fmt/base.h> enum class color {red, green, blue}; template <> struct fmt::formatter<color>: formatter<string_view> { // parse is inherited from formatter<string_view>. auto format(color c, format_context& ctx) const -> format_context::iterator; }; </code></pre> <pre><code class="language-c++">// color.cc: #include "color.h" #include <fmt/format.h> auto fmt::formatter<color>::format(color c, format_context& ctx) const -> format_context::iterator { string_view name = "unknown"; switch (c) { case color::red: name = "red"; break; case color::green: name = "green"; break; case color::blue: name = "blue"; break; } return formatter<string_view>::format(name, ctx); } </code></pre> <p>Note that <code>formatter<string_view>::format</code> is defined in <code>fmt/format.h</code> so it has to be included in the source file. Since <code>parse</code> is inherited from <code>formatter<string_view></code> it will recognize all string format specifications, for example</p> <pre><code class="language-c++">fmt::format("{:>10}", color::blue) </code></pre> <p>will return <code>" blue"</code>.</p> <p>In general the formatter has the following form:</p> <pre class="highlight"><code>template <> struct fmt::formatter<T> { // Parses format specifiers and stores them in the formatter. // // [ctx.begin(), ctx.end()) is a, possibly empty, character range that // contains a part of the format string starting from the format // specifications to be parsed, e.g. in // // fmt::format("{:f} continued", ...); // // the range will contain "f} continued". The formatter should parse // specifiers until '}' or the end of the range. In this example the // formatter should parse the 'f' specifier and return an iterator // pointing to '}'. constexpr auto parse(format_parse_context& ctx) -> format_parse_context::iterator; // Formats value using the parsed format specification stored in this // formatter and writes the output to ctx.out(). auto format(const T& value, format_context& ctx) const -> format_context::iterator; };</code></pre> <p>It is recommended to at least support fill, align and width that apply to the whole object and have the same semantics as in standard formatters.</p> <p>You can also write a formatter for a hierarchy of classes:</p> <pre><code class="language-c++">// demo.h: #include <type_traits> #include <fmt/core.h> struct A { virtual ~A() {} virtual std::string name() const { return "A"; } }; struct B : A { virtual std::string name() const { return "B"; } }; template <typename T> struct fmt::formatter<T, std::enable_if_t<std::is_base_of_v<A, T>, char>> : fmt::formatter<std::string> { auto format(const A& a, format_context& ctx) const { return formatter<std::string>::format(a.name(), ctx); } }; </code></pre> <pre><code class="language-c++">// demo.cc: #include "demo.h" #include <fmt/format.h> int main() { B b; A& a = b; fmt::print("{}", a); // Output: B } </code></pre> <p>Providing both a <code>formatter</code> specialization and a <code>format_as</code> overload is disallowed.</p> <div class="docblock"><a id="basic_format_parse_context"> <pre><code class="language-cpp decl"> <div>template <typename Char> </div> <div>using basic_format_parse_context = parse_context<Char>; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="context"> <pre><code class="language-cpp decl"> <div></div> <div>class context; </div></code></pre></a> <div class="docblock-desc"> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>context(iterator out, format_args args, detail::locale_ref loc); </div></code></pre> <div class="docblock-desc"> <p>Constructs a <code>context</code> object. References to the arguments are stored in the object so make sure they have appropriate lifetimes.</p> </div> </div> </div> </div> <div class="docblock"><a id="format_context"> <pre><code class="language-cpp decl"> <div></div> <div>using format_context = context; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <h3 id="compile-time-checks">Compile-Time Checks</h3> <p>Compile-time format string checks are enabled by default on compilers that support C++20 <code>consteval</code>. On older compilers you can use the <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#legacy-checks">FMT_STRING</a> macro defined in <code>fmt/format.h</code> instead.</p> <p>Unused arguments are allowed as in Python's <code>str.format</code> and ordinary functions.</p> <div class="docblock"><a id="basic_format_string"> <pre><code class="language-cpp decl"> <div>template <typename Char, typename... T> </div> <div>using basic_format_string = basic_fstring<Char, T...>; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="format_string"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>using format_string = typename fstring<T...>::t; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="runtime"> <pre><code class="language-cpp decl"> <div></div> <div>auto runtime(string_view s) -⁠> runtime_format_string<>; </div></code></pre></a> <div class="docblock-desc"> <p>Creates a runtime format string.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">// Check format string at runtime instead of compile-time. fmt::print(fmt::runtime("{:d}"), "I am not a number"); </code></pre> <p></p> </div> </div> <h3 id="named-arguments">Named Arguments</h3> <div class="docblock"><a id="arg"> <pre><code class="language-cpp decl"> <div>template <typename Char, typename T> </div> <div>auto arg(const Char* name, const T& arg) -⁠> detail::named_arg<Char, T>; </div></code></pre></a> <div class="docblock-desc"> <p>Returns a named argument to be used in a formatting function. It should only be used in a call to a formatting function.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print("The answer is {answer}.", fmt::arg("answer", 42)); </code></pre> <p></p> </div> </div> <p>Named arguments are not supported in compile-time checks at the moment.</p> <h3 id="type-erasure">Type Erasure</h3> <p>You can create your own formatting function with compile-time checks and small binary footprint, for example (<a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://godbolt.org/z/b9Pbasvzc">run</a>):</p> <pre><code class="language-c++">#include <fmt/format.h> void vlog(const char* file, int line, fmt::string_view fmt, fmt::format_args args) { fmt::print("{}: {}: {}", file, line, fmt::vformat(fmt, args)); } template <typename... T> void log(const char* file, int line, fmt::format_string<T...> fmt, T&&... args) { vlog(file, line, fmt, fmt::make_format_args(args...)); } #define MY_LOG(fmt, ...) log(__FILE__, __LINE__, fmt, __VA_ARGS__) MY_LOG("invalid squishiness: {}", 42); </code></pre> <p>Note that <code>vlog</code> is not parameterized on argument types which improves compile times and reduces binary code size compared to a fully parameterized version.</p> <div class="docblock"><a id="make_format_args"> <pre><code class="language-cpp decl"> <div>template <typename Context, typename... T, int NUM_ARGS, int NUM_NAMED_ARGS, unsigned long long DESC> </div> <div>constexpr auto make_format_args(T&... args) -⁠> detail::format_arg_store<Context, NUM_ARGS, NUM_NAMED_ARGS, DESC>; </div></code></pre></a> <div class="docblock-desc"> <p>Constructs an object that stores references to arguments and can be implicitly converted to <code>format_args</code>. <code>Context</code> can be omitted in which case it defaults to <code>context</code>. See <code>arg</code> for lifetime considerations.</p> </div> </div> <div class="docblock"><a id="basic_format_args"> <pre><code class="language-cpp decl"> <div>template <typename Context> </div> <div>class basic_format_args; </div></code></pre></a> <div class="docblock-desc"> <p>A view of a collection of formatting arguments. To avoid lifetime issues it should only be used as a parameter type in type-erased functions such as <code>vformat</code>:</p> <pre><code class="language-cpp">void vlog(fmt::string_view fmt, fmt::format_args args); // OK fmt::format_args args = fmt::make_format_args(); // Dangling reference </code></pre> <p></p> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>constexpr basic_format_args(const store<NUM_ARGS, NUM_NAMED_ARGS, DESC>& s); </div></code></pre> <div class="docblock-desc"> <p>Constructs a <code>basic_format_args</code> object from <code>format_arg_store</code>.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>constexpr basic_format_args(const format_arg* args, int count, bool has_named); </div></code></pre> <div class="docblock-desc"> <p>Constructs a <code>basic_format_args</code> object from a dynamic list of arguments.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>auto get(int id) -⁠> format_arg; </div></code></pre> <div class="docblock-desc"> <p>Returns the argument with the specified id.</p> </div> </div> </div> </div> <div class="docblock"><a id="format_args"> <pre><code class="language-cpp decl"> <div></div> <div>using format_args = basic_format_args<context>; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="basic_format_arg"> <pre><code class="language-cpp decl"> <div>template <typename Context> </div> <div>class basic_format_arg; </div></code></pre></a> <div class="docblock-desc"> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>auto visit(Visitor&& vis) -⁠> decltype(vis(0)); </div></code></pre> <div class="docblock-desc"> <p>Visits an argument dispatching to the appropriate visit method based on the argument type. For example, if the argument type is <code>double</code> then <code>vis(value)</code> will be called with the value of type <code>double</code>.</p> </div> </div> </div> </div> <h3 id="compatibility">Compatibility</h3> <div class="docblock"><a id="basic_string_view"> <pre><code class="language-cpp decl"> <div>template <typename Char> </div> <div>class basic_string_view; </div></code></pre></a> <div class="docblock-desc"> <p>An implementation of <code>std::basic_string_view</code> for pre-C++17. It provides a subset of the API. <code>fmt::basic_string_view</code> is used for format strings even if <code>std::basic_string_view</code> is available to prevent issues when a library is compiled with a different <code>-std</code> option than the client code (which is not recommended).</p> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>constexpr basic_string_view(const Char* s, size_t count); </div></code></pre> <div class="docblock-desc"> <p>Constructs a string reference object from a C string and a size.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>basic_string_view(const Char* s); </div></code></pre> <div class="docblock-desc"> <p>Constructs a string reference object from a C string.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>basic_string_view(const S& s); </div></code></pre> <div class="docblock-desc"> <p>Constructs a string reference from a <code>std::basic_string</code> or a <code>std::basic_string_view</code> object.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>constexpr auto data() -⁠> const Char*; </div></code></pre> <div class="docblock-desc"> <p>Returns a pointer to the string data.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>constexpr auto size() -⁠> size_t; </div></code></pre> <div class="docblock-desc"> <p>Returns the string size.</p> </div> </div> </div> </div> <div class="docblock"><a id="string_view"> <pre><code class="language-cpp decl"> <div></div> <div>using string_view = basic_string_view<char>; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <h2 id="format-api">Format API</h2> <p><code>fmt/format.h</code> defines the full format API providing additional formatting functions and locale support.</p> <p><a id="format"></a></p> <div class="docblock"><a id="format"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>auto format(format_string<T...> fmt, T&&... args) -⁠> std::string; </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and returns the result as a string.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">#include <fmt/format.h> std::string message = fmt::format("The answer is {}.", 42); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="vformat"> <pre><code class="language-cpp decl"> <div></div> <div>auto vformat(string_view fmt, format_args args) -⁠> std::string; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="operator" _a> <pre><code class="language-cpp decl"> <div>template <detail::fixed_string S> </div> <div>constexpr auto operator""_a(); </div></code></pre></a> <div class="docblock-desc"> </div> </div> <h3 id="utilities">Utilities</h3> <div class="docblock"><a id="ptr"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>auto ptr(T p) -⁠> const void*; </div></code></pre></a> <div class="docblock-desc"> <p>Converts <code>p</code> to <code>const void*</code> for pointer formatting.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">auto s = fmt::format("{}", fmt::ptr(p)); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="underlying"> <pre><code class="language-cpp decl"> <div>template <typename Enum> </div> <div>constexpr auto underlying(Enum e) -⁠> underlying_t<Enum>; </div></code></pre></a> <div class="docblock-desc"> <p>Converts <code>e</code> to the underlying type.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">enum class color { red, green, blue }; auto s = fmt::format("{}", fmt::underlying(color::red)); // s == "0" </code></pre> <p></p> </div> </div> <div class="docblock"><a id="to_string"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>auto to_string(const T& value) -⁠> std::string; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="group_digits"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>auto group_digits(T value) -⁠> group_digits_view<T>; </div></code></pre></a> <div class="docblock-desc"> <p>Returns a view that formats an integer value using ',' as a locale-independent thousands separator.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print("{}", fmt::group_digits(12345)); // Output: "12,345" </code></pre> <p></p> </div> </div> <div class="docblock"><a id="detail::buffer"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>class detail::buffer; </div></code></pre></a> <div class="docblock-desc"> <p>A contiguous memory buffer with an optional growing ability. It is an internal class and shouldn't be used directly, only via <code>memory_buffer</code>.</p> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>constexpr auto size() -⁠> size_t; </div></code></pre> <div class="docblock-desc"> <p>Returns the size of this buffer.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>constexpr auto capacity() -⁠> size_t; </div></code></pre> <div class="docblock-desc"> <p>Returns the capacity of this buffer.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>auto data() -⁠> T*; </div></code></pre> <div class="docblock-desc"> <p>Returns a pointer to the buffer data (not null-terminated).</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void clear(); </div></code></pre> <div class="docblock-desc"> <p>Clears this buffer.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void append(const U* begin, const U* end); </div></code></pre> <div class="docblock-desc"> <p>Appends data to the end of the buffer.</p> </div> </div> </div> </div> <div class="docblock"><a id="basic_memory_buffer"> <pre><code class="language-cpp decl"> <div>template <typename T, size_t SIZE, typename Allocator> </div> <div>class basic_memory_buffer; </div></code></pre></a> <div class="docblock-desc"> <p>A dynamically growing memory buffer for trivially copyable/constructible types with the first <code>SIZE</code> elements stored in the object itself. Most commonly used via the <code>memory_buffer</code> alias for <code>char</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">auto out = fmt::memory_buffer(); fmt::format_to(std::back_inserter(out), "The answer is {}.", 42); </code></pre> This will append "The answer is 42." to <code>out</code>. The buffer content can be converted to <code>std::string</code> with <code>to_string(out)</code>. <p></p> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>basic_memory_buffer(basic_memory_buffer&& other); </div></code></pre> <div class="docblock-desc"> <p>Constructs a <code>basic_memory_buffer</code> object moving the content of the other object to it.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>auto operator=(basic_memory_buffer&& other) -⁠> basic_memory_buffer&; </div></code></pre> <div class="docblock-desc"> <p>Moves the content of the other <code>basic_memory_buffer</code> object to this one.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void resize(size_t count); </div></code></pre> <div class="docblock-desc"> <p>Resizes the buffer to contain <code>count</code> elements. If T is a POD type new elements may not be initialized.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void reserve(size_t new_capacity); </div></code></pre> <div class="docblock-desc"> <p>Increases the buffer capacity to <code>new_capacity</code>.</p> </div> </div> </div> </div> <h3 id="system-errors">System Errors</h3> <p>{fmt} does not use <code>errno</code> to communicate errors to the user, but it may call system functions which set <code>errno</code>. Users should not make any assumptions about the value of <code>errno</code> being preserved by library functions.</p> <div class="docblock"><a id="system_error"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>auto system_error(int error_code, format_string<T...> fmt, T&&... args) -⁠> std::system_error; </div></code></pre></a> <div class="docblock-desc"> <p>Constructs <code>std::system_error</code> with a message formatted with <code>fmt::format(fmt, args...)</code>. <code>error_code</code> is a system error code as given by <code>errno</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">// This throws std::system_error with the description // cannot open file 'madeup': No such file or directory // or similar (system message may vary). const char* filename = "madeup"; FILE* file = fopen(filename, "r"); if (!file) throw fmt::system_error(errno, "cannot open file '{}'", filename); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="format_system_error"> <pre><code class="language-cpp decl"> <div></div> <div>void format_system_error(detail::buffer<char>& out, int error_code, const char* message); </div></code></pre></a> <div class="docblock-desc"> <p>Formats an error message for an error returned by an operating system or a language runtime, for example a file opening error, and writes it to <code>out</code>. The format is the same as the one used by <code>std::system_error(ec, message)</code> where <code>ec</code> is <code>std::error_code(error_code, std::generic_category())</code>. It is implementation-defined but normally looks like:</p> <pre><code class="language-cpp"><message>: <system-message> </code></pre> where <code><message></code> is the passed message and <code><system-message></code> is the system message corresponding to the error code. <code>error_code</code> is a system error code as given by <code>errno</code>. <p></p> </div> </div> <h3 id="custom-allocators">Custom Allocators</h3> <p>The {fmt} library supports custom dynamic memory allocators. A custom allocator class can be specified as a template argument to <a href="https://fmt-dev.translate.goog/latest/api/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#basic_memory_buffer"><code>fmt::basic_memory_buffer</code></a>:</p> <pre class="highlight"><code>using custom_memory_buffer = fmt::basic_memory_buffer<char, fmt::inline_buffer_size, custom_allocator>;</code></pre> <p>It is also possible to write a formatting function that uses a custom allocator:</p> <pre class="highlight"><code>using custom_string = std::basic_string<char, std::char_traits<char>, custom_allocator>; auto vformat(custom_allocator alloc, fmt::string_view fmt, fmt::format_args args) -> custom_string { auto buf = custom_memory_buffer(alloc); fmt::vformat_to(std::back_inserter(buf), fmt, args); return custom_string(buf.data(), buf.size(), alloc); } template <typename ...Args> auto format(custom_allocator alloc, fmt::string_view fmt, const Args& ... args) -> custom_string { return vformat(alloc, fmt, fmt::make_format_args(args...)); }</code></pre> <p>The allocator will be used for the output container only. Formatting functions normally don't do any allocations for built-in and string types except for non-default floating-point formatting that occasionally falls back on <code>sprintf</code>.</p> <h3 id="locale">Locale</h3> <p>All formatting is locale-independent by default. Use the <code>'L'</code> format specifier to insert the appropriate number separator characters from the locale:</p> <pre class="highlight"><code>#include <fmt/core.h> #include <locale> std::locale::global(std::locale("en_US.UTF-8")); auto s = fmt::format("{:L}", 1000000); // s == "1,000,000"</code></pre> <p><code>fmt/format.h</code> provides the following overloads of formatting functions that take <code>std::locale</code> as a parameter. The locale type is a template parameter to avoid the expensive <code><locale></code> include.</p> <div class="docblock"><a id="format"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>auto format(detail::locale_ref loc, format_string<T...> fmt, T&&... args) -⁠> std::string; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="format_to"> <pre><code class="language-cpp decl"> <div>template <typename OutputIt, typename... T> </div> <div>auto format_to(OutputIt out, detail::locale_ref loc, format_string<T...> fmt, T&&... args) -⁠> OutputIt; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="formatted_size"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>auto formatted_size(detail::locale_ref loc, format_string<T...> fmt, T&&... args) -⁠> size_t; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <p><a id="legacy-checks"></a></p> <h3 id="legacy-compile-time-checks">Legacy Compile-Time Checks</h3> <p><code>FMT_STRING</code> enables compile-time checks on older compilers. It requires C++14 or later and is a no-op in C++11.</p> <div class="docblock"><a id="FMT_STRING"> <pre><code class="language-cpp decl"> <div></div> <div>FMT_STRING(s) </div></code></pre></a> <div class="docblock-desc"> <p>Constructs a legacy compile-time format string from a string literal <code>s</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">// A compile-time error because 'd' is an invalid specifier for strings. std::string s = fmt::format(FMT_STRING("{:d}"), "foo"); </code></pre> <p></p> </div> </div> <p>To force the use of legacy compile-time checks, define the preprocessor variable <code>FMT_ENFORCE_COMPILE_STRING</code>. When set, functions accepting <code>FMT_STRING</code> will fail to compile with regular strings.</p> <p><a id="ranges-api"></a></p> <h2 id="range-and-tuple-formatting">Range and Tuple Formatting</h2> <p><code>fmt/ranges.h</code> provides formatting support for ranges and tuples:</p> <pre class="highlight"><code>#include <fmt/ranges.h> fmt::print("{}", std::tuple<char, int>{'a', 42}); // Output: ('a', 42)</code></pre> <p>Using <code>fmt::join</code>, you can separate tuple elements with a custom separator:</p> <pre class="highlight"><code>#include <fmt/ranges.h> auto t = std::tuple<int, char>{1, 'a'}; fmt::print("{}", fmt::join(t, ", ")); // Output: 1, a</code></pre> <div class="docblock"><a id="join"> <pre><code class="language-cpp decl"> <div>template <typename Range> </div> <div>auto join(Range&& r, string_view sep) -⁠> join_view<decltype(detail::range_begin(r)), decltype(detail::range_end(r))>; </div></code></pre></a> <div class="docblock-desc"> <p>Returns a view that formats <code>range</code> with elements separated by <code>sep</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">auto v = std::vector<int>{1, 2, 3}; fmt::print("{}", fmt::join(v, ", ")); // Output: 1, 2, 3 </code></pre><code>fmt::join</code> applies passed format specifiers to the range elements: <pre><code class="language-cpp">fmt::print("{:02}", fmt::join(v, ", ")); // Output: 01, 02, 03 </code></pre> <p></p> </div> </div> <div class="docblock"><a id="join"> <pre><code class="language-cpp decl"> <div>template <typename It, typename Sentinel> </div> <div>auto join(It begin, Sentinel end, string_view sep) -⁠> join_view<It, Sentinel>; </div></code></pre></a> <div class="docblock-desc"> <p>Returns a view that formats the iterator range <code>[begin, end)</code> with elements separated by <code>sep</code>.</p> </div> </div> <div class="docblock"><a id="join"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>auto join(std::initializer_list<T> list, string_view sep) -⁠> join_view<const T*, const T*>; </div></code></pre></a> <div class="docblock-desc"> <p>Returns an object that formats <code>std::initializer_list</code> with elements separated by <code>sep</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print("{}", fmt::join({1, 2, 3}, ", ")); // Output: "1, 2, 3" </code></pre> <p></p> </div> </div> <p><a id="chrono-api"></a></p> <h2 id="date-and-time-formatting">Date and Time Formatting</h2> <p><code>fmt/chrono.h</code> provides formatters for</p> <ul> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/chrono/duration"><code>std::chrono::duration</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/chrono/time_point"><code>std::chrono::time_point</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/chrono/c/tm"><code>std::tm</code></a></li> </ul> <p>The format syntax is described in <a href="https://fmt-dev.translate.goog/latest/syntax/?_x_tr_sl=pl&_x_tr_tl=iw&_x_tr_hl=en-GB#chrono-format-specifications">Chrono Format Specifications</a>.</p> <p><strong>Example</strong>:</p> <pre class="highlight"><code>#include <fmt/chrono.h> int main() { std::time_t t = std::time(nullptr); fmt::print("The date is {:%Y-%m-%d}.", fmt::localtime(t)); // Output: The date is 2020-11-07. // (with 2020-11-07 replaced by the current date) using namespace std::literals::chrono_literals; fmt::print("Default format: {} {}\n", 42s, 100ms); // Output: Default format: 42s 100ms fmt::print("strftime-like format: {:%H:%M:%S}\n", 3h + 15min + 30s); // Output: strftime-like format: 03:15:30 }</code></pre> <div class="docblock"><a id="localtime"> <pre><code class="language-cpp decl"> <div></div> <div>auto localtime(std::time_t time) -⁠> std::tm; </div></code></pre></a> <div class="docblock-desc"> <p>Converts given time since epoch as <code>std::time_t</code> value into calendar time, expressed in local time. Unlike <code>std::localtime</code>, this function is thread-safe on most platforms.</p> </div> </div> <div class="docblock"><a id="gmtime"> <pre><code class="language-cpp decl"> <div></div> <div>auto gmtime(std::time_t time) -⁠> std::tm; </div></code></pre></a> <div class="docblock-desc"> <p>Converts given time since epoch as <code>std::time_t</code> value into calendar time, expressed in Coordinated Universal Time (UTC). Unlike <code>std::gmtime</code>, this function is thread-safe on most platforms.</p> </div> </div> <p><a id="std-api"></a></p> <h2 id="standard-library-types-formatting">Standard Library Types Formatting</h2> <p><code>fmt/std.h</code> provides formatters for:</p> <ul> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/atomic/atomic"><code>std::atomic</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/atomic/atomic_flag"><code>std::atomic_flag</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/utility/bitset"><code>std::bitset</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/error/error_code"><code>std::error_code</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/error/exception"><code>std::exception</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/filesystem/path"><code>std::filesystem::path</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/utility/variant/monostate"><code>std::monostate</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/utility/optional"><code>std::optional</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/utility/source_location"><code>std::source_location</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/thread/thread/id"><code>std::thread::id</code></a></li> <li><a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/utility/variant/variant"><code>std::variant</code></a></li> </ul> <div class="docblock"><a id="ptr"> <pre><code class="language-cpp decl"> <div>template <typename T, typename Deleter> </div> <div>auto ptr(const std::unique_ptr<T, Deleter>& p) -⁠> const void*; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="ptr"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>auto ptr(const std::shared_ptr<T>& p) -⁠> const void*; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <h3 id="variants">Variants</h3> <p>A <code>std::variant</code> is only formattable if every variant alternative is formattable, and requires the <code>__cpp_lib_variant</code> <a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/feature_test">library feature</a>.</p> <p><strong>Example</strong>:</p> <pre class="highlight"><code>#include <fmt/std.h> fmt::print("{}", std::variant<char, float>('x')); // Output: variant('x') fmt::print("{}", std::variant<std::monostate, char>()); // Output: variant(monostate)</code></pre> <h2 id="bit-fields-and-packed-structs">Bit-Fields and Packed Structs</h2> <p>To format a bit-field or a field of a struct with <code>__attribute__((packed))</code> applied to it, you need to convert it to the underlying or compatible type via a cast or a unary <code>+</code> (<a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://www.godbolt.org/z/3qKKs6T5Y">godbolt</a>):</p> <pre><code class="language-c++">struct smol { int bit : 1; }; auto s = smol(); fmt::print("{}", +s.bit); </code></pre> <p>This is a known limitation of "perfect" forwarding in C++.</p> <p><a id="compile-api"></a></p> <h2 id="format-string-compilation">Format String Compilation</h2> <p><code>fmt/compile.h</code> provides format string compilation enabled via the <code>FMT_COMPILE</code> macro or the <code>_cf</code> user-defined literal defined in namespace <code>fmt::literals</code>. Format strings marked with <code>FMT_COMPILE</code> or <code>_cf</code> are parsed, checked and converted into efficient formatting code at compile-time. This supports arguments of built-in and string types as well as user-defined types with <code>format</code> functions taking the format context type as a template parameter in their <code>formatter</code> specializations. For example:</p> <pre class="highlight"><code>template <> struct fmt::formatter<point> { constexpr auto parse(format_parse_context& ctx); template <typename FormatContext> auto format(const point& p, FormatContext& ctx) const; };</code></pre> <p>Format string compilation can generate more binary code compared to the default API and is only recommended in places where formatting is a performance bottleneck.</p> <div class="docblock"><a id="FMT_COMPILE"> <pre><code class="language-cpp decl"> <div></div> <div>FMT_COMPILE(s) </div></code></pre></a> <div class="docblock-desc"> <p>Converts a string literal <code>s</code> into a format string that will be parsed at compile time and converted into efficient formatting code. Requires C++17 <code>constexpr if</code> compiler support.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">// Converts 42 into std::string using the most efficient method and no // runtime format string processing. std::string s = fmt::format(FMT_COMPILE("{}"), 42); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="operator" _cf> <pre><code class="language-cpp decl"> <div>template <detail::fixed_string Str> </div> <div>constexpr auto operator""_cf(); </div></code></pre></a> <div class="docblock-desc"> </div> </div> <p><a id="color-api"></a></p> <h2 id="terminal-colors-and-text-styles">Terminal Colors and Text Styles</h2> <p><code>fmt/color.h</code> provides support for terminal color and text style output.</p> <div class="docblock"><a id="print"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>void print(const text_style& ts, format_string<T...> fmt, T&&... args); </div></code></pre></a> <div class="docblock-desc"> <p>Formats a string and prints it to stdout using ANSI escape sequences to specify text formatting.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print(fmt::emphasis::bold | fg(fmt::color::red), "Elapsed time: {0:.2f} seconds", 1.23); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="fg"> <pre><code class="language-cpp decl"> <div></div> <div>auto fg(detail::color_type foreground) -⁠> text_style; </div></code></pre></a> <div class="docblock-desc"> <p>Creates a text style from the foreground (text) color.</p> </div> </div> <div class="docblock"><a id="bg"> <pre><code class="language-cpp decl"> <div></div> <div>auto bg(detail::color_type background) -⁠> text_style; </div></code></pre></a> <div class="docblock-desc"> <p>Creates a text style from the background color.</p> </div> </div> <div class="docblock"><a id="styled"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>auto styled(const T& value, text_style ts) -⁠> detail::styled_arg<remove_cvref_t<T>>; </div></code></pre></a> <div class="docblock-desc"> <p>Returns an argument that will be formatted using ANSI escape sequences, to be used in a formatting function.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print("Elapsed time: {0:.2f} seconds", fmt::styled(1.23, fmt::fg(fmt::color::green) | fmt::bg(fmt::color::blue))); </code></pre> <p></p> </div> </div> <p><a id="os-api"></a></p> <h2 id="system-apis">System APIs</h2> <div class="docblock"><a id="ostream"> <pre><code class="language-cpp decl"> <div></div> <div>class ostream; </div></code></pre></a> <div class="docblock-desc"> <p>A fast buffered output stream for writing from a single thread. Writing from multiple threads without external synchronization may result in a data race.</p> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void print(format_string<T...> fmt, T&&... args); </div></code></pre> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and writes the output to the file.</p> </div> </div> </div> </div> <div class="docblock"><a id="windows_error"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>auto windows_error(int error_code, string_view message, const T&... args) -⁠> std::system_error; </div></code></pre></a> <div class="docblock-desc"> <p>Constructs a <code>std::system_error</code> object with the description of the form</p> <pre><code class="language-cpp"><message>: <system-message> </code></pre> where <code><message></code> is the formatted message and <code><system-message></code> is the system message corresponding to the error code. <code>error_code</code> is a Windows error code as given by <code>GetLastError</code>. If <code>error_code</code> is not a valid error code such as -1, the system message will look like "error -1". <p></p> <p><b>Example</b>:</p> <pre><code class="language-cpp">// This throws a system_error with the description // cannot open file 'madeup': The system cannot find the file </code></pre> specified. // or similar (system message may vary). const char *filename = "madeup"; LPOFSTRUCT of = LPOFSTRUCT(); HFILE file = OpenFile(filename, &of, OF_READ); if (file == HFILE_ERROR) { throw fmt::windows_error(GetLastError(), "cannot open file '{}'", filename); } <p></p> </div> </div> <p><a id="ostream-api"></a></p> <h2 id="stdostream-support"><code>std::ostream</code> Support</h2> <p><code>fmt/ostream.h</code> provides <code>std::ostream</code> support including formatting of user-defined types that have an overloaded insertion operator (<code>operator<<</code>). In order to make a type formattable via <code>std::ostream</code> you should provide a <code>formatter</code> specialization inherited from <code>ostream_formatter</code>:</p> <pre class="highlight"><code>#include <fmt/ostream.h> struct date { int year, month, day; friend std::ostream& operator<<(std::ostream& os, const date& d) { return os << d.year << '-' << d.month << '-' << d.day; } }; template <> struct fmt::formatter<date> : ostream_formatter {}; std::string s = fmt::format("The date is {}", date{2012, 12, 9}); // s == "The date is 2012-12-9"</code></pre> <div class="docblock"><a id="streamed"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>constexpr auto streamed(const T& value) -⁠> detail::streamed_view<T>; </div></code></pre></a> <div class="docblock-desc"> <p>Returns a view that formats <code>value</code> via an ostream <code>operator<<</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print("Current thread id: {}\n", fmt::streamed(std::this_thread::get_id())); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="print"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>void print(std::ostream& os, format_string<T...> fmt, T&&... args); </div></code></pre></a> <div class="docblock-desc"> <p>Prints formatted data to the stream <code>os</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::print(cerr, "Don't {}!", "panic"); </code></pre> <p></p> </div> </div> <p><a id="args-api"></a></p> <h2 id="dynamic-argument-lists">Dynamic Argument Lists</h2> <p>The header <code>fmt/args.h</code> provides <code>dynamic_format_arg_store</code>, a builder-like API that can be used to construct format argument lists dynamically.</p> <div class="docblock"><a id="dynamic_format_arg_store"> <pre><code class="language-cpp decl"> <div>template <typename Context> </div> <div>class dynamic_format_arg_store; </div></code></pre></a> <div class="docblock-desc"> <p>A dynamic list of formatting arguments with storage.</p> <p>It can be implicitly converted into <code>fmt::basic_format_args</code> for passing into type-erased formatting functions such as <code>fmt::vformat</code>.</p> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void push_back(const T& arg); </div></code></pre> <div class="docblock-desc"> <p>Adds an argument into the dynamic store for later passing to a formatting function.</p> <p>Note that custom types and string types (but not string views) are copied into the store dynamically allocating memory if necessary.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::dynamic_format_arg_store<fmt::format_context> store; store.push_back(42); store.push_back("abc"); store.push_back(1.5f); std::string result = fmt::vformat("{} and {} and {}", store); </code></pre> <p></p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void push_back(std::reference_wrapper<T> arg); </div></code></pre> <div class="docblock-desc"> <p>Adds a reference to the argument into the dynamic store for later passing to a formatting function.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::dynamic_format_arg_store<fmt::format_context> store; char band[] = "Rolling Stones"; store.push_back(std::cref(band)); band[9] = 'c'; // Changing str affects the output. std::string result = fmt::vformat("{}", store); // result == "Rolling Scones" </code></pre> <p></p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void push_back(const detail::named_arg<char_type, T>& arg); </div></code></pre> <div class="docblock-desc"> <p>Adds named argument into the dynamic store for later passing to a formatting function. <code>std::reference_wrapper</code> is supported to avoid copying of the argument. The name is always copied into the store.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void clear(); </div></code></pre> <div class="docblock-desc"> <p>Erase all elements from the store.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>void reserve(size_t new_cap, size_t new_cap_named); </div></code></pre> <div class="docblock-desc"> <p>Reserves space to store at least <code>new_cap</code> arguments including <code>new_cap_named</code> named arguments.</p> </div> </div> <div class="docblock"> <pre><code class="language-cpp decl"> <div></div> <div>size_t size(); </div></code></pre> <div class="docblock-desc"> <p>Returns the number of elements in the store.</p> </div> </div> </div> </div> <p><a id="printf-api"></a></p> <h2 id="safe-printf">Safe <code>printf</code></h2> <p>The header <code>fmt/printf.h</code> provides <code>printf</code>-like formatting functionality. The following functions use <a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html">printf format string syntax</a> with the POSIX extension for positional arguments. Unlike their standard counterparts, the <code>fmt</code> functions are type-safe and throw an exception if an argument type doesn't match its format specification.</p> <div class="docblock"><a id="printf"> <pre><code class="language-cpp decl"> <div>template <typename... T> </div> <div>auto printf(string_view fmt, const T&... args) -⁠> int; </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and writes the output to <code>stdout</code>.</p> <p><b>Example</b>:</p> <p>fmt::printf("Elapsed time: %.2f seconds", 1.23);</p> </div> </div> <div class="docblock"><a id="fprintf"> <pre><code class="language-cpp decl"> <div>template <typename S, typename... T, typename Char> </div> <div>auto fprintf(std::FILE* f, const S& fmt, const T&... args) -⁠> int; </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and writes the output to <code>f</code>.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">fmt::fprintf(stderr, "Don't %s!", "panic"); </code></pre> <p></p> </div> </div> <div class="docblock"><a id="sprintf"> <pre><code class="language-cpp decl"> <div>template <typename S, typename... T, typename Char> </div> <div>auto sprintf(const S& fmt, const T&... args) -⁠> std::basic_string<Char>; </div></code></pre></a> <div class="docblock-desc"> <p>Formats <code>args</code> according to specifications in <code>fmt</code> and returns the result as as string.</p> <p><b>Example</b>:</p> <pre><code class="language-cpp">std::string message = fmt::sprintf("The answer is %d", 42); </code></pre> <p></p> </div> </div> <p><a id="xchar-api"></a></p> <h2 id="wide-strings">Wide Strings</h2> <p>The optional header <code>fmt/xchar.h</code> provides support for <code>wchar_t</code> and exotic character types.</p> <div class="docblock"><a id="is_char"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>struct is_char; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="wstring_view"> <pre><code class="language-cpp decl"> <div></div> <div>using wstring_view = basic_string_view<wchar_t>; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="wformat_context"> <pre><code class="language-cpp decl"> <div></div> <div>using wformat_context = buffered_context<wchar_t>; </div></code></pre></a> <div class="docblock-desc"> </div> </div> <div class="docblock"><a id="to_wstring"> <pre><code class="language-cpp decl"> <div>template <typename T> </div> <div>auto to_wstring(const T& value) -⁠> std::wstring; </div></code></pre></a> <div class="docblock-desc"> <p>Converts <code>value</code> to <code>std::wstring</code> using the default format for type <code>T</code>.</p> </div> </div> <h2 id="compatibility-with-c20-stdformat">Compatibility with C++20 <code>std::format</code></h2> <p>{fmt} implements nearly all of the <a href="https://translate.google.com/website?sl=pl&tl=iw&hl=en-GB&u=https://en.cppreference.com/w/cpp/utility/format">C++20 formatting library</a> with the following differences:</p> <ul> <li>Names are defined in the <code>fmt</code> namespace instead of <code>std</code> to avoid collisions with standard library implementations.</li> <li>Width calculation doesn't use grapheme clusterization. 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API - {fmt}