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26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="tribool.tutorial"></a>Tutorial</h2></div></div></div>
28<div class="toc"><dl>
29<dt><span class="section"><a href="tutorial.html#id1816834">Basic usage</a></span></dt>
30<dt><span class="section"><a href="tutorial.html#id1817306">Renaming the indeterminate state</a></span></dt>
31<dt><span class="section"><a href="tutorial.html#id1817393"><code class="computeroutput">tribool</code> input/output</a></span></dt>
32</dl></div>
33<div class="section" lang="en">
34<div class="titlepage"><div><div><h3 class="title">
35<a name="id1816834"></a>Basic usage</h3></div></div></div>
36<p> The <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class acts
37    like the built-in <code class="computeroutput">bool</code> type, but for 3-state boolean
38    logic. The three states are <code class="computeroutput">true</code>, <code class="computeroutput">false</code>,
39    and <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>, where
40    the first two states are equivalent to those of the C++
41    <code class="computeroutput">bool</code> type and the last state represents an unknown
42    boolean value (that may be <code class="computeroutput">true</code> or
43    <code class="computeroutput">false</code>, we don't know).</p>
44<p> The <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class
45    supports conversion from <code class="computeroutput">bool</code> values and literals
46    along with its own
47    <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>
48    keyword:</p>
49<pre class="programlisting"><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b(true);
50b = false;
51b = <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>;
52<code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b2(b);</pre>
53<p> <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports
54    conversions to <code class="computeroutput">bool</code> for use in conditional
55    statements. The conversion to <code class="computeroutput">bool</code> will be
56    <code class="computeroutput">true</code> when the value of the
57    <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is always true, and
58    <code class="computeroutput">false</code> otherwise. Consequently, the following idiom
59    may be used to determine which of the three states a
60    <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> currently
61    holds:</p>
62<pre class="programlisting"><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b = some_operation();
63if (b) {
64  // b is true
65}
66else if (!b) {
67  // b is false
68}
69else {
70  // b is indeterminate
71}</pre>
72<p> <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports the
73  3-state logic operators <code class="computeroutput">!</code> (negation),
74  <code class="computeroutput">&amp;&amp;</code> (AND), and <code class="computeroutput">||</code> (OR), with
75  <code class="computeroutput">bool</code> and <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>
76  values. For instance:</p>
77<pre class="programlisting"><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = some_op();
78<code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> y = some_other_op();
79if (x &amp;&amp; y) {
80  // both x and y are true
81}
82else if (!(x &amp;&amp; y)) {
83  // either x or y is false
84}
85else {
86  // neither x nor y is false, but we don't know that both are true
87
88  if (x || y) {
89    // either x or y is true
90  }
91}</pre>
92<p> Similarly, <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>
93  supports 3-state equality comparisons via the operators
94  <code class="computeroutput">==</code> and <code class="computeroutput">!=</code>. These operators differ from
95  "normal" equality operators in C++ because they return a
96  <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>, because potentially we
97  might not know the result of a comparison (try to compare
98  <code class="computeroutput">true</code> and
99  <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>). For
100  instance:</p>
101<pre class="programlisting"><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(true);
102<code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> y(<code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>);
103
104assert(x == x); // okay, x == x returns true
105assert(x == true); // okay, can compare <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>s and bools</pre>
106<p> The <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> keyword (representing the
107  <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> value)
108  doubles as a function to check if the value of a
109  <code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is indeterminate,
110  e.g.,</p>
111<pre class="programlisting"><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = try_to_do_something_tricky();
112if (<code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>(x)) {
113  // value of x is indeterminate
114}
115else {
116  // report success or failure of x
117}</pre>
118</div>
119<div class="section" lang="en">
120<div class="titlepage"><div><div><h3 class="title">
121<a name="id1817306"></a>Renaming the indeterminate state</h3></div></div></div>
122<p> Users may introduce additional keywords for the indeterminate
123  value in addition to the implementation-supplied
124  <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> using the
125  <code class="computeroutput"><a href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code>
126  macro. For instance, the following macro instantiation (at the
127  global scope) will introduce the keyword <code class="computeroutput">maybe</code> as a
128  synonym for <code class="computeroutput"><a href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>
129  (also residing in the <code class="computeroutput">boost</code> namespace):</p>
130<pre class="programlisting"><code class="computeroutput"><a href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code>(maybe)
131<code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = maybe;
132if (maybe(x)) { /* ... */ }</pre>
133</div>
134<div class="section" lang="en">
135<div class="titlepage"><div><div><h3 class="title">
136<a name="id1817393"></a><code class="computeroutput">tribool</code> input/output</h3></div></div></div>
137<p><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> objects may be
138  read from and written to streams by including the
139  <code class="computeroutput"><a 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
140  manner very similar to <code class="computeroutput">bool</code> values. When the
141  <code class="computeroutput">boolalpha</code> flag is not set on the input/output stream,
142  the integral values 0, 1, and 2 correspond to <code class="computeroutput">tribool</code>
143  values <code class="computeroutput">false</code>, <code class="computeroutput">true</code>, and
144  <code class="computeroutput">indeterminate</code>, respectively. When
145  <code class="computeroutput">boolalpha</code> is set on the stream, arbitrary strings can
146  be used to represent the three values, the default being "false",
147  "true", and "indeterminate". For instance:</p>
148<pre class="programlisting"><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x;
149cin &gt;&gt; x; // Type "0", "1", or "2" to get false, true, or indeterminate
150cout &lt;&lt; boolalpha &lt;&lt; x; // Produces "false", "true", or "indeterminate"</pre>
151<p><code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> input and output
152  is sensitive to the stream's current locale. The strings associated
153  with false and true values are contained in the standard
154  <code class="computeroutput">std::numpunct</code> facet, and the
155  string naming the indeterminate type is contained in the
156  <code class="computeroutput"><a href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code> facet. To
157  replace the name of the indeterminate state, you need to imbue your
158  stream with a local containing a
159  <code class="computeroutput"><a href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code> facet, e.g.:</p>
160<pre class="programlisting"><code class="computeroutput"><a href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code>(maybe)
161locale global;
162locale test_locale(global, new <code class="computeroutput"><a href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code>&lt;char&gt;("maybe"));
163cout.imbue(test_locale);
164<code class="computeroutput"><a href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(maybe);
165cout &lt;&lt; boolalpha &lt;&lt; x &lt;&lt; endl; // Prints "maybe"</pre>
166<p>If you C++ standard library implementation does not support
167  locales, <code class="computeroutput">tribool</code> input/output will still work, but you
168  will be unable to customize the strings printed/parsed when
169  <code class="computeroutput">boolalpha</code> is set.</p>
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