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*: transfer back to cmake from autotools autotools just made lots of things more complicated than they should have and many things broke (i.e. translations)
author Paper <paper@paper.us.eu.org>
date Thu, 20 Jun 2024 05:56:06 -0400
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125 <h1>{fmt}</h1>
126 <p class="lead">A modern formatting library</p>
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162 <section id="overview">
163 <h1>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</a></h1>
164 <p><strong>{fmt}</strong> is an open-source formatting library providing a fast and safe
165 alternative to C stdio and C++ iostreams.</p>
166 <div class="panel panel-default">
167 <div class="panel-heading">What users say:</div>
168 <div class="panel-body">
169 Thanks for creating this library. It’s been a hole in C++ for
170 a long time. I’ve used both <code>boost::format</code> and
171 <code>loki::SPrintf</code>, and neither felt like the right answer.
172 This does.
173 </div>
174 </div><section id="format-api">
175 <span id="format-api-intro"></span><h2>Format API<a class="headerlink" href="#format-api" title="Permalink to this headline">¶</a></h2>
176 <p>The format API is similar in spirit to the C <code class="docutils literal notranslate"><span class="pre">printf</span></code> family of function but
177 is safer, simpler and several times <a class="reference external" href="https://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html">faster</a>
178 than common standard library implementations.
179 The <a class="reference external" href="syntax.html">format string syntax</a> is similar to the one used by
180 <a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#str.format">str.format</a> in
181 Python:</p>
182 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">fmt</span><span class="o">::</span><span class="n">format</span><span class="p">(</span><span class="s">&quot;The answer is {}.&quot;</span><span class="p">,</span><span class="w"> </span><span class="mi">42</span><span class="p">);</span>
183 </pre></div>
184 </div>
185 <p>The <code class="docutils literal notranslate"><span class="pre">fmt::format</span></code> function returns a string “The answer is 42.”. You can use
186 <code class="docutils literal notranslate"><span class="pre">fmt::memory_buffer</span></code> to avoid constructing <code class="docutils literal notranslate"><span class="pre">std::string</span></code>:</p>
187 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="k">auto</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">fmt</span><span class="o">::</span><span class="n">memory_buffer</span><span class="p">();</span>
188 <span class="n">fmt</span><span class="o">::</span><span class="n">format_to</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">back_inserter</span><span class="p">(</span><span class="n">out</span><span class="p">),</span>
189 <span class="w"> </span><span class="s">&quot;For a moment, {} happened.&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;nothing&quot;</span><span class="p">);</span>
190 <span class="k">auto</span><span class="w"> </span><span class="n">data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">out</span><span class="p">.</span><span class="n">data</span><span class="p">();</span><span class="w"> </span><span class="c1">// pointer to the formatted data</span>
191 <span class="k">auto</span><span class="w"> </span><span class="n">size</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">out</span><span class="p">.</span><span class="n">size</span><span class="p">();</span><span class="w"> </span><span class="c1">// size of the formatted data</span>
192 </pre></div>
193 </div>
194 <p>The <code class="docutils literal notranslate"><span class="pre">fmt::print</span></code> function performs formatting and writes the result to a stream:</p>
195 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">fmt</span><span class="o">::</span><span class="n">print</span><span class="p">(</span><span class="n">stderr</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;System error code = {}</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">errno</span><span class="p">);</span>
196 </pre></div>
197 </div>
198 <p>If you omit the file argument the function will print to <code class="docutils literal notranslate"><span class="pre">stdout</span></code>:</p>
199 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">fmt</span><span class="o">::</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;Don&#39;t {}</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;panic&quot;</span><span class="p">);</span>
200 </pre></div>
201 </div>
202 <p>The format API also supports positional arguments useful for localization:</p>
203 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">fmt</span><span class="o">::</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;I&#39;d rather be {1} than {0}.&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;right&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;happy&quot;</span><span class="p">);</span>
204 </pre></div>
205 </div>
206 <p>You can pass named arguments with <code class="docutils literal notranslate"><span class="pre">fmt::arg</span></code>:</p>
207 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">fmt</span><span class="o">::</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;Hello, {name}! The answer is {number}. Goodbye, {name}.&quot;</span><span class="p">,</span>
208 <span class="w"> </span><span class="n">fmt</span><span class="o">::</span><span class="n">arg</span><span class="p">(</span><span class="s">&quot;name&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;World&quot;</span><span class="p">),</span><span class="w"> </span><span class="n">fmt</span><span class="o">::</span><span class="n">arg</span><span class="p">(</span><span class="s">&quot;number&quot;</span><span class="p">,</span><span class="w"> </span><span class="mi">42</span><span class="p">));</span>
209 </pre></div>
210 </div>
211 <p>If your compiler supports C++11 user-defined literals, the suffix <code class="docutils literal notranslate"><span class="pre">_a</span></code> offers
212 an alternative, slightly terser syntax for named arguments:</p>
213 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="k">using</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="nn">fmt</span><span class="o">::</span><span class="nn">literals</span><span class="p">;</span>
214 <span class="n">fmt</span><span class="o">::</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;Hello, {name}! The answer is {number}. Goodbye, {name}.&quot;</span><span class="p">,</span>
215 <span class="w"> </span><span class="s">&quot;name&quot;</span><span class="n">_a</span><span class="o">=</span><span class="s">&quot;World&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;number&quot;</span><span class="n">_a</span><span class="o">=</span><span class="mi">42</span><span class="p">);</span>
216 </pre></div>
217 </div>
218 </section>
219 <section id="safety">
220 <span id="id1"></span><h2>Safety<a class="headerlink" href="#safety" title="Permalink to this headline">¶</a></h2>
221 <p>The library is fully type safe, automatic memory management prevents buffer
222 overflow, errors in format strings are reported using exceptions or at compile
223 time. For example, the code</p>
224 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">fmt</span><span class="o">::</span><span class="n">format</span><span class="p">(</span><span class="s">&quot;The answer is {:d}&quot;</span><span class="p">,</span><span class="w"> </span><span class="s">&quot;forty-two&quot;</span><span class="p">);</span>
225 </pre></div>
226 </div>
227 <p>throws the <code class="docutils literal notranslate"><span class="pre">format_error</span></code> exception because the argument <code class="docutils literal notranslate"><span class="pre">&quot;forty-two&quot;</span></code> is a
228 string while the format code <code class="docutils literal notranslate"><span class="pre">d</span></code> only applies to integers.</p>
229 <p>The code</p>
230 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">format</span><span class="p">(</span><span class="n">FMT_STRING</span><span class="p">(</span><span class="s">&quot;The answer is {:d}&quot;</span><span class="p">),</span><span class="w"> </span><span class="s">&quot;forty-two&quot;</span><span class="p">);</span>
231 </pre></div>
232 </div>
233 <p>reports a compile-time error on compilers that support relaxed <code class="docutils literal notranslate"><span class="pre">constexpr</span></code>.
234 See <a class="reference external" href="api.html#compile-time-format-string-checks">here</a> for details.</p>
235 <p>The following code</p>
236 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span>fmt::format(&quot;Cyrillic letter {}&quot;, L&#39;\x42e&#39;);
237 </pre></div>
238 </div>
239 <p>produces a compile-time error because wide character <code class="docutils literal notranslate"><span class="pre">L'\x42e'</span></code> cannot be
240 formatted into a narrow string. For comparison, writing a wide character to
241 <code class="docutils literal notranslate"><span class="pre">std::ostream</span></code> results in its numeric value being written to the stream
242 (i.e. 1070 instead of letter ‘ю’ which is represented by <code class="docutils literal notranslate"><span class="pre">L'\x42e'</span></code> if we
243 use Unicode) which is rarely desirable.</p>
244 </section>
245 <section id="compact-binary-code">
246 <h2>Compact Binary Code<a class="headerlink" href="#compact-binary-code" title="Permalink to this headline">¶</a></h2>
247 <p>The library produces compact per-call compiled code. For example
248 (<a class="reference external" href="https://godbolt.org/g/TZU4KF">godbolt</a>),</p>
249 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;fmt/core.h&gt;</span>
250
251 <span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
252 <span class="w"> </span><span class="n">fmt</span><span class="o">::</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;The answer is {}.&quot;</span><span class="p">,</span><span class="w"> </span><span class="mi">42</span><span class="p">);</span>
253 <span class="p">}</span>
254 </pre></div>
255 </div>
256 <p>compiles to just</p>
257 <div class="highlight-asm notranslate"><div class="highlight"><pre><span></span>main: # @main
258 sub rsp, 24
259 mov qword ptr [rsp], 42
260 mov rcx, rsp
261 mov edi, offset .L.str
262 mov esi, 17
263 mov edx, 1
264 call fmt::v7::vprint(fmt::v7::basic_string_view&lt;char&gt;, fmt::v7::format_args)
265 xor eax, eax
266 add rsp, 24
267 ret
268 .L.str:
269 .asciz &quot;The answer is {}.&quot;
270 </pre></div>
271 </div>
272 </section>
273 <section id="portability">
274 <span id="id2"></span><h2>Portability<a class="headerlink" href="#portability" title="Permalink to this headline">¶</a></h2>
275 <p>The library is highly portable and relies only on a small set of C++11 features:</p>
276 <ul class="simple">
277 <li><p>variadic templates</p></li>
278 <li><p>type traits</p></li>
279 <li><p>rvalue references</p></li>
280 <li><p>decltype</p></li>
281 <li><p>trailing return types</p></li>
282 <li><p>deleted functions</p></li>
283 <li><p>alias templates</p></li>
284 </ul>
285 <p>These are available in GCC 4.8, Clang 3.4, MSVC 19.0 (2015) and more recent
286 compiler version. For older compilers use {fmt} <a class="reference external" href="https://github.com/fmtlib/fmt/releases/tag/4.1.0">version 4.x</a> which is maintained and
287 only requires C++98.</p>
288 <p>The output of all formatting functions is consistent across platforms.
289 For example,</p>
290 <div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">fmt</span><span class="o">::</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;{}&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">numeric_limits</span><span class="o">&lt;</span><span class="kt">double</span><span class="o">&gt;::</span><span class="n">infinity</span><span class="p">());</span>
291 </pre></div>
292 </div>
293 <p>always prints <code class="docutils literal notranslate"><span class="pre">inf</span></code> while the output of <code class="docutils literal notranslate"><span class="pre">printf</span></code> is platform-dependent.</p>
294 </section>
295 <section id="ease-of-use">
296 <span id="id3"></span><h2>Ease of Use<a class="headerlink" href="#ease-of-use" title="Permalink to this headline">¶</a></h2>
297 <p>{fmt} has a small self-contained code base with the core library consisting of
298 just three header files and no external dependencies.
299 A permissive MIT <a class="reference external" href="https://github.com/fmtlib/fmt#license">license</a> allows
300 using the library both in open-source and commercial projects.</p>
301 <p><a class="reference external" href="contents.html">Learn more…</a></p>
302 <a class="btn btn-success" href="https://github.com/fmtlib/fmt">GitHub Repository</a>
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