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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="tribool.tutorial"></a>Tutorial</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.2">Basic usage</a></span></dt>
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<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.3">Renaming the indeterminate state</a></span></dt>
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<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.4"><code class="computeroutput">tribool</code> input/output</a></span></dt>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id-1.3.24.4.2"></a>Basic usage</h3></div></div></div>
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<p> The <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class acts
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like the built-in <code class="computeroutput">bool</code> type, but for 3-state boolean
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logic. The three states are <code class="computeroutput">true</code>, <code class="computeroutput">false</code>,
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and <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>, where
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the first two states are equivalent to those of the C++
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<code class="computeroutput">bool</code> type and the last state represents an unknown
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boolean value (that may be <code class="computeroutput">true</code> or
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<code class="computeroutput">false</code>, we don't know).</p>
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<p> The <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class
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supports conversion from <code class="computeroutput">bool</code> values and literals
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along with its own
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<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>
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keyword:</p>
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<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b(true);
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b = false;
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b = <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>;
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b2(b);</pre>
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<p> <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports
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conversions to <code class="computeroutput">bool</code> for use in conditional
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statements. The conversion to <code class="computeroutput">bool</code> will be
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<code class="computeroutput">true</code> when the value of the
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is always true, and
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<code class="computeroutput">false</code> otherwise. Consequently, the following idiom
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may be used to determine which of the three states a
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> currently
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holds:</p>
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<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b = some_operation();
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if (b) {
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// b is true
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}
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else if (!b) {
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// b is false
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}
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else {
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// b is indeterminate
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}</pre>
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<p> <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports the
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3-state logic operators <code class="computeroutput">!</code> (negation),
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<code class="computeroutput">&&</code> (AND), and <code class="computeroutput">||</code> (OR), with
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<code class="computeroutput">bool</code> and <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>
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values. For instance:</p>
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<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = some_op();
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> y = some_other_op();
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if (x && y) {
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// both x and y are true
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}
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else if (!(x && y)) {
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// either x or y is false
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}
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else {
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// neither x nor y is false, but we don't know that both are true
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if (x || y) {
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// either x or y is true
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}
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}</pre>
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<p> Similarly, <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>
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supports 3-state equality comparisons via the operators
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<code class="computeroutput">==</code> and <code class="computeroutput">!=</code>. These operators differ from
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"normal" equality operators in C++ because they return a
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>, because potentially we
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might not know the result of a comparison (try to compare
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<code class="computeroutput">true</code> and
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<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>). For
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instance:</p>
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<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(true);
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<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>);
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assert(x == x); // okay, x == x returns true
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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>
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<p> The <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> keyword (representing the
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<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)
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doubles as a function to check if the value of a
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is indeterminate,
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e.g.,</p>
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<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();
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if (<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>(x)) {
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// value of x is indeterminate
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}
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else {
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// report success or failure of x
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}</pre>
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<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
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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
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be used in compile time expressions:</p>
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<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));
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<code class="computeroutput">static_assert</code>(x, "Must be true!");
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</pre>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<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>
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</table></div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id-1.3.24.4.3"></a>Renaming the indeterminate state</h3></div></div></div>
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<p> Users may introduce additional keywords for the indeterminate
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value in addition to the implementation-supplied
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<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> using the
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<code class="computeroutput"><a class="link" href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code>
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macro. For instance, the following macro instantiation (at the
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global scope) will introduce the keyword <code class="computeroutput">maybe</code> as a
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synonym for <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>
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(also residing in the <code class="computeroutput">boost</code> namespace):</p>
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<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)
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = maybe;
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if (maybe(x)) { /* ... */ }</pre>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id-1.3.24.4.4"></a><code class="computeroutput">tribool</code> input/output</h3></div></div></div>
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<p><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> objects may be
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read from and written to streams by including the
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<code class="computeroutput"><a class="link" href="reference.html#header.boost.logic.tribool_io_hpp" title="Header <boost/logic/tribool_io.hpp>">boost/logic/tribool_io.hpp</a></code> header in a
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manner very similar to <code class="computeroutput">bool</code> values. When the
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<code class="computeroutput">boolalpha</code> flag is not set on the input/output stream,
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the integral values 0, 1, and 2 correspond to <code class="computeroutput">tribool</code>
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values <code class="computeroutput">false</code>, <code class="computeroutput">true</code>, and
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<code class="computeroutput">indeterminate</code>, respectively. When
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<code class="computeroutput">boolalpha</code> is set on the stream, arbitrary strings can
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be used to represent the three values, the default being "false",
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"true", and "indeterminate". For instance:</p>
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<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x;
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cin >> x; // Type "0", "1", or "2" to get false, true, or indeterminate
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cout << boolalpha << x; // Produces "false", "true", or "indeterminate"</pre>
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<p><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> input and output
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is sensitive to the stream's current locale. The strings associated
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with false and true values are contained in the standard
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<code class="computeroutput">std::numpunct</code> facet, and the
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string naming the indeterminate type is contained in the
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<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code> facet. To
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replace the name of the indeterminate state, you need to imbue your
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stream with a local containing a
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<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>
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<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)
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locale global;
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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><char>("maybe"));
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cout.imbue(test_locale);
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<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(maybe);
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cout << boolalpha << x << endl; // Prints "maybe"</pre>
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<p>If you C++ standard library implementation does not support
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locales, <code class="computeroutput">tribool</code> input/output will still work, but you
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will be unable to customize the strings printed/parsed when
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<code class="computeroutput">boolalpha</code> is set.</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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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
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Software License, Version 1.0. (See accompanying file
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<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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</div></td>
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</tr></table>
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<a accesskey="p" href="../tribool.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../tribool.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="reference.html"><img src="../../../doc/src/images/next.png" alt="Next"></a>
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