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[DEV] add v1.76.0
2021-10-05 21:37:46 +02:00
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<a name="tribool.tutorial"></a>Tutorial</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.2">Basic usage</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.3">Renaming the indeterminate state</a></span></dt>
<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.4"><code class="computeroutput">tribool</code> input/output</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.24.4.2"></a>Basic usage</h3></div></div></div>
<p> The <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class acts
like the built-in <code class="computeroutput">bool</code> type, but for 3-state boolean
logic. The three states are <code class="computeroutput">true</code>, <code class="computeroutput">false</code>,
and <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>, where
the first two states are equivalent to those of the C++
<code class="computeroutput">bool</code> type and the last state represents an unknown
boolean value (that may be <code class="computeroutput">true</code> or
<code class="computeroutput">false</code>, we don't know).</p>
<p> The <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class
supports conversion from <code class="computeroutput">bool</code> values and literals
along with its own
<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>
keyword:</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b(true);
b = false;
b = <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>;
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b2(b);</pre>
<p> <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports
conversions to <code class="computeroutput">bool</code> for use in conditional
statements. The conversion to <code class="computeroutput">bool</code> will be
<code class="computeroutput">true</code> when the value of the
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is always true, and
<code class="computeroutput">false</code> otherwise. Consequently, the following idiom
may be used to determine which of the three states a
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> currently
holds:</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b = some_operation();
if (b) {
// b is true
}
else if (!b) {
// b is false
}
else {
// b is indeterminate
}</pre>
<p> <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports the
3-state logic operators <code class="computeroutput">!</code> (negation),
<code class="computeroutput">&amp;&amp;</code> (AND), and <code class="computeroutput">||</code> (OR), with
<code class="computeroutput">bool</code> and <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>
values. For instance:</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = some_op();
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> y = some_other_op();
if (x &amp;&amp; y) {
// both x and y are true
}
else if (!(x &amp;&amp; y)) {
// either x or y is false
}
else {
// neither x nor y is false, but we don't know that both are true
if (x || y) {
// either x or y is true
}
}</pre>
<p> Similarly, <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>
supports 3-state equality comparisons via the operators
<code class="computeroutput">==</code> and <code class="computeroutput">!=</code>. These operators differ from
"normal" equality operators in C++ because they return a
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>, because potentially we
might not know the result of a comparison (try to compare
<code class="computeroutput">true</code> and
<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>). For
instance:</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(true);
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> y(<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>);
assert(x == x); // okay, x == x returns true
assert(x == true); // okay, can compare <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>s and bools</pre>
<p> The <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> keyword (representing the
<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> value)
doubles as a function to check if the value of a
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is indeterminate,
e.g.,</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = try_to_do_something_tricky();
if (<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>(x)) {
// value of x is indeterminate
}
else {
// report success or failure of x
}</pre>
<p> All the logical operators and methods of <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> are marked
as <code class="computeroutput">constexpr</code> in C++11. It means that <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> can
be used in compile time expressions:</p>
<pre class="programlisting">constexpr <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = (tribool(true) || tribool(indeterminate));
<code class="computeroutput">static_assert</code>(x, "Must be true!");
</pre>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">Some compilers may have troubles with evaluating <code class="computeroutput">tribool::operator safe_bool()</code> at compile time.</td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.24.4.3"></a>Renaming the indeterminate state</h3></div></div></div>
<p> Users may introduce additional keywords for the indeterminate
value in addition to the implementation-supplied
<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> using the
<code class="computeroutput"><a class="link" href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code>
macro. For instance, the following macro instantiation (at the
global scope) will introduce the keyword <code class="computeroutput">maybe</code> as a
synonym for <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>
(also residing in the <code class="computeroutput">boost</code> namespace):</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code>(maybe)
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = maybe;
if (maybe(x)) { /* ... */ }</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id-1.3.24.4.4"></a><code class="computeroutput">tribool</code> input/output</h3></div></div></div>
<p><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> objects may be
read from and written to streams by including the
<code class="computeroutput"><a class="link" href="reference.html#header.boost.logic.tribool_io_hpp" title="Header &lt;boost/logic/tribool_io.hpp&gt;">boost/logic/tribool_io.hpp</a></code> header in a
manner very similar to <code class="computeroutput">bool</code> values. When the
<code class="computeroutput">boolalpha</code> flag is not set on the input/output stream,
the integral values 0, 1, and 2 correspond to <code class="computeroutput">tribool</code>
values <code class="computeroutput">false</code>, <code class="computeroutput">true</code>, and
<code class="computeroutput">indeterminate</code>, respectively. When
<code class="computeroutput">boolalpha</code> is set on the stream, arbitrary strings can
be used to represent the three values, the default being "false",
"true", and "indeterminate". For instance:</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x;
cin &gt;&gt; x; // Type "0", "1", or "2" to get false, true, or indeterminate
cout &lt;&lt; boolalpha &lt;&lt; x; // Produces "false", "true", or "indeterminate"</pre>
<p><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> input and output
is sensitive to the stream's current locale. The strings associated
with false and true values are contained in the standard
<code class="computeroutput">std::numpunct</code> facet, and the
string naming the indeterminate type is contained in the
<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code> facet. To
replace the name of the indeterminate state, you need to imbue your
stream with a local containing a
<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code> facet, e.g.:</p>
<pre class="programlisting"><code class="computeroutput"><a class="link" href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code>(maybe)
locale global;
locale test_locale(global, new <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code>&lt;char&gt;("maybe"));
cout.imbue(test_locale);
<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(maybe);
cout &lt;&lt; boolalpha &lt;&lt; x &lt;&lt; endl; // Prints "maybe"</pre>
<p>If you C++ standard library implementation does not support
locales, <code class="computeroutput">tribool</code> input/output will still work, but you
will be unable to customize the strings printed/parsed when
<code class="computeroutput">boolalpha</code> is set.</p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2004 Douglas Gregor<p>Use, modification and distribution is subject to the Boost
Software License, Version 1.0. (See accompanying file
<code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p>
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