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<td >Qt 5.9</td><td ><a href="qtcore-index.html">Qt Core</a></td><td >File and Datastream Functions</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right">Qt 5.9.4 Reference Documentation</td>
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<h1 class="title">File and Datastream Functions</h1>
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<p>The <a href="qiodevice.html">QIODevice</a> class is the base interface class of all I/O devices in <a href="qtcore-index.html">Qt Core</a>. <a href="qiodevice.html">QIODevice</a> provides both a common implementation and an abstract interface for devices that support reading and writing of blocks of data. The device can be a memory buffer, a file, or a datastream.</p>
<p>Some subclasses like <a href="qfile.html">QFile</a> have been implemented using a memory buffer for intermediate storing of data. This speeds up programs by reducing read/write operations. Buffering makes functions like <a href="qiodevice.html#getChar">getChar()</a> and <a href="qiodevice.html#putChar">putChar()</a> fast, as they can operate on the memory buffer instead of directly on the device itself.</p>
<p>The <a href="qfile.html">QFile</a> class provides functions for reading from and writing to files. A <a href="qfile.html">QFile</a> may be used by itself or, more conveniently, with a <a href="qtextstream.html">QTextStream</a> or <a href="qdatastream.html">QDataStream</a>.</p>
<p><a href="qbuffer.html">QBuffer</a> allows you to access a <a href="qbytearray.html">QByteArray</a> using the <a href="qiodevice.html">QIODevice</a> interface. The <a href="qbytearray.html">QByteArray</a> is treated just as a standard random-accessed file. An example:</p>
<pre class="cpp">
<span class="type"><a href="qbuffer.html">QBuffer</a></span> buffer;
<span class="type">char</span> ch;
buffer<span class="operator">.</span>open(<span class="type"><a href="qbuffer.html">QBuffer</a></span><span class="operator">::</span>ReadWrite);
buffer<span class="operator">.</span>write(<span class="string">"Qt rocks!"</span>);
buffer<span class="operator">.</span>seek(<span class="number">0</span>);
buffer<span class="operator">.</span>getChar(<span class="operator">&</span>ch); <span class="comment">// ch == 'Q'</span>
buffer<span class="operator">.</span>getChar(<span class="operator">&</span>ch); <span class="comment">// ch == 't'</span>
buffer<span class="operator">.</span>getChar(<span class="operator">&</span>ch); <span class="comment">// ch == ' '</span>
buffer<span class="operator">.</span>getChar(<span class="operator">&</span>ch); <span class="comment">// ch == 'r'</span>
</pre>
<p>Call <a href="qbuffer.html#open">open()</a> to open the buffer. Then call <a href="qiodevice.html#write">write()</a> or <a href="qiodevice.html#putChar">putChar()</a> to write to the buffer, and <a href="qiodevice.html#read">read()</a>, <a href="qiodevice.html#readLine">readLine()</a>, <a href="qiodevice.html#readAll">readAll()</a>, or <a href="qiodevice.html#getChar">getChar()</a> to read from it. <a href="qbuffer.html#size">size()</a> returns the current size of the buffer, and you can seek to arbitrary positions in the buffer by calling <a href="qbuffer.html#seek">seek()</a>. When you are done with accessing the buffer, call <a href="qbuffer.html#close">close()</a>.</p>
<p>The <a href="qdatastream.html">QDataStream</a> class provides serialization of binary data to a <a href="qiodevice.html">QIODevice</a>. A data stream is a binary stream of encoded information which is 100% inde- pendent of the host computer's operating system, CPU or byte order. For example, a data stream that is written by a PC under Windows can be read by a Sun SPARC running Solaris. You can also use a data stream to read/write raw unencoded binary data.</p>
<p>For more details on the datatypes that <a href="qdatastream.html">QDataStream</a> can serialize, see <a href="datastreamformat.html">Serializing Qt Data Types</a>.</p>
<p>The <a href="qtextstream.html">QTextStream</a> class provides a convenient interface for reading and writing text. <a href="qtextstream.html">QTextStream</a> can operate on a <a href="qiodevice.html">QIODevice</a>, a <a href="qbytearray.html">QByteArray</a> or a <a href="qstring.html">QString</a>. Using <a href="qtextstream.html">QTextStream</a>'s streaming operators, you can conveniently read and write words, lines and numbers. It's also common to use <a href="qtextstream.html">QTextStream</a> to read console input and write console output.</p>
<p>There are three general ways to use <a href="qtextstream.html">QTextStream</a> when reading text files:</p>
<ul>
<li>Chunk by chunk, by calling <a href="qiodevice.html#readLine">readLine()</a> or <a href="qiodevice.html#readAll">readAll()</a>.</li>
<li>Word by word. <a href="qtextstream.html">QTextStream</a> supports streaming into <a href="qstring.html">QString</a>s, <a href="qbytearray.html">QByteArray</a>s and char* buffers. Words are delimited by space, and leading white space is automatically skipped.</li>
<li>Character by character, by streaming into <a href="qchar.html">QChar</a> or char types. This method is often used for convenient input handling when parsing files, independent of character encoding and end-of-line semantics. To skip white space, call <a href="qtextstream.html#skipWhiteSpace">skipWhiteSpace()</a>.</li>
</ul>
<p><a href="qbytearray.html">QByteArray</a> can be used to store both raw bytes (including <code>\0</code>) and traditional 8-bit '\0'-terminated strings. Using <a href="qbytearray.html">QByteArray</a> is much more convenient than using const char *. It always ensures that the data is followed by a '\0' terminator, and uses <a href="implicit-sharing.html">implicitly shared classes</a> (copy-on-write) to reduce memory usage and avoid needless copying of data.</p>
<p>In addition to <a href="qbytearray.html">QByteArray</a>, Qt also provides the <a href="qstring.html">QString</a> class to store string data. For most purposes, <a href="qstring.html">QString</a> is the most appropriate class to use. It stores 16-bit Unicode characters. It is, however, a good idea to use <a href="qbytearray.html">QByteArray</a> when you need to store raw binary data, and when memory conservation is critical (for example, with Qt for Embedded Linux).</p>
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