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 </code></pre></div> <p>When defined to <code>1</code>, parsing from a <code class=highlight><span class=n>std</span><span class=o>::</span><span class=n>istream</span></code> reads the input one character at a time through the stream buffer, as in version 3.12.0 and earlier, instead of scanning the characters that the stream buffer already holds in bulk.</p> <p>The macro is an escape hatch for stream buffers that do not follow the usual contract of <code>std::basic_streambuf</code> (see the requirements on the stream buffer in the notes below). It does not change any result: values, exceptions, and the position of the stream after parsing are the same either way. Only the speed differs.</p> <h2 id=default-definition>Default definition<a class=headerlink href=#default-definition title="Permanent link">&para;</a></h2> <p>The default value is <code>0</code>: streams are scanned in bulk.</p> <div class=highlight><pre><span></span><code><span class=cp>#define JSON_DISABLE_STREAM_BULK_SCAN 0</span>
 </code></pre></div> <h2 id=notes>Notes<a class=headerlink href=#notes title="Permanent link">&para;</a></h2> <div class="admonition note"> <p class=admonition-title>How streams are scanned</p> <p>A <code>std::streambuf</code> keeps the part of the input it has read but not yet handed out in its <em>get area</em>, the range from <code>gptr()</code> to <code>egptr()</code>. A <code>std::filebuf</code>, for instance, reads the file in blocks of a few kilobytes into its get area, and a <code>std::stringbuf</code> exposes its whole string as one get area.</p> <p>When parsing from a <code>std::istream</code>, the parser scans this get area directly, with the same fast paths that it uses for a <code>std::string</code>: runs of ordinary string characters, numbers, and indentation are recognized several bytes at a time instead of being requested from the stream buffer one character at a time. When the parser reaches the end of the get area, it asks the stream buffer for the next character as before, which makes the stream buffer refill its get area.</p> <p>The parser only reads characters that it would also have read one at a time, and it advances the stream buffer exactly over the characters it consumes. Nothing is read ahead, buffered elsewhere, or put back. Therefore, the stream is left exactly where it was left before:</p> <ul> <li><a href=../../operator_gtgt/ ><code>operator&gt;&gt;</code></a> and <a href=../../basic_json/sax_parse/ ><code>sax_parse</code></a> with <code>strict = false</code> can still read several values from one stream, and the stream can still be used with other <code>std::istream</code> functions after a value has been read.</li> <li>With <a href=../json_precise_stream_position/ ><code>JSON_PRECISE_STREAM_POSITION</code></a>, the stream is still positioned right after a parsed number.</li> <li>Exception messages, including the line, the column, and the last read token, are unchanged.</li> </ul> </div> <div class="admonition note"> <p class=admonition-title>Performance</p> <p>Parsing from a <code>std::ifstream</code> or <code>std::istringstream</code> is typically 1.5 to 2 times as fast as before, and reading many small values from one stream with <code>operator&gt;&gt;</code> 1.4 to 4 times as fast. Parsing from a stream is then within about 10 to 20 % of first reading the whole file into a <code>std::string</code> and parsing that.</p> <p>A stream buffer without a get area (one that only overrides <code>underflow()</code> and <code>uflow()</code> and hands out every character from there) offers nothing to scan in bulk, so it is read one character at a time as before.</p> </div> <div class="admonition note"> <p class=admonition-title>Requirements on the stream buffer</p> <p>The bulk scan relies on the following behavior, which every stream buffer of the standard library and every stream buffer that keeps its input in its get area has:</p> <ul> <li>When <code>sgetc()</code> returns a character while <code>gptr() &lt; egptr()</code>, that character is <code>*gptr()</code>; when <code>sbumpc()</code> returns one, it was at <code>gptr()</code> before the call.</li> <li>Characters in the get area are not changed while they are in the get area.</li> <li><code>underflow()</code> or <code>uflow()</code> either sets up a get area that holds the returned character at <code>gptr()</code> (or at <code>gptr() - 1</code> after <code>uflow()</code>), or leaves the get area empty.</li> </ul> <p>A stream buffer that returns characters from <code>uflow()</code> while leaving unrelated characters in its get area, or that changes the characters in its get area in place, does not meet these requirements. Define the macro to <code>1</code> for such stream buffers.</p> </div> <div class="admonition note"> <p class=admonition-title>ABI compatibility</p> <p>The value of this macro is encoded in the <a href=../../../features/namespace/ >namespace</a> (tag <code>_dsbs</code>), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.</p> </div> <div class="admonition note"> <p class=admonition-title>Other inputs</p> <p>Strings, string views, vectors, arrays, and pointer or iterator ranges over contiguous memory are scanned in bulk anyway. Reading from a <code class=highlight><span class=kt>FILE</span><span class=o>*</span></code> is not affected by this macro; see <a href=../json_disable_unlocked_file_io/ ><code>JSON_DISABLE_UNLOCKED_FILE_IO</code></a> for that input.</p> </div> <h2 id=examples>Examples<a class=headerlink href=#examples title="Permanent link">&para;</a></h2> <details class=example> <summary>Example: reading values from a stream (default)</summary> <p>The stream is scanned in bulk, and the values are read one after the other:</p> <div class=highlight><pre><span></span><code><span class=cp>#include</span><span class=w> </span><span clLine truncated
 <span class=cp>#include</span><span class=w> </span><span class=cpf>&lt;sstream&gt;</span>