source: downloads/boost_1_34_1/libs/logic/doc/tribool.boostbook @ 29

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE library PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
  "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<library name="Tribool" dirname="logic" id="tribool"
         last-revision="$Date: 2004/07/25 03:01:38 $" xmlns:xi="http://www.w3.org/2001/XInclude">
  <libraryinfo>
    <author>
      <firstname>Douglas</firstname>
      <surname>Gregor</surname>
      <email>dgregor -at- cs.indiana.edu</email>
    </author>

    <copyright>
      <year>2002</year>
      <year>2003</year>
      <year>2004</year>
      <holder>Douglas Gregor</holder>
    </copyright>    

    <legalnotice>
      <para>Use, modification and distribution is subject to the Boost
    Software License, Version 1.0. (See accompanying file
    <filename>LICENSE_1_0.txt</filename> or copy at <ulink
    url="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</ulink>)</para>
    </legalnotice>

    <librarypurpose>Three-state boolean type</librarypurpose>
    <librarycategory name="category:misc"/>
  </libraryinfo>

  <title>Boost.Tribool</title>

  <section id="tribool.introduction">
    <title>Introduction</title>

    <para>The 3-state boolean library contains a single class,
    <code><classname>boost::logic::tribool</classname></code>, along with
    support functions and operator overloads that implement 3-state
    boolean logic. </para>
  </section>

  <section id="tribool.tutorial">
    <title>Tutorial</title>

    <using-namespace name="boost::logic"/>

    <section>
      <title>Basic usage</title>
    <para> The <code><classname>tribool</classname></code> class acts
    like the built-in <code>bool</code> type, but for 3-state boolean
    logic. The three states are <code>true</code>, <code>false</code>,
    and <code><functionname>indeterminate</functionname></code>, where
    the first two states are equivalent to those of the C++
    <code>bool</code> type and the last state represents an unknown
    boolean value (that may be <code>true</code> or
    <code>false</code>, we don't know).</para>

    <para> The <code><classname>tribool</classname></code> class
    supports conversion from <code>bool</code> values and literals
    along with its own
    <code><functionname>indeterminate</functionname></code>
    keyword:</para>

    <programlisting><classname>tribool</classname> b(true);
b = false;
b = <functionname>indeterminate</functionname>;
<classname>tribool</classname> b2(b);</programlisting>

    <para> <code><classname>tribool</classname></code> supports
    conversions to <code>bool</code> for use in conditional
    statements. The conversion to <code>bool</code> will be
    <code>true</code> when the value of the
    <code><classname>tribool</classname></code> is always true, and
    <code>false</code> otherwise. Consequently, the following idiom
    may be used to determine which of the three states a
    <code><classname>tribool</classname></code> currently
    holds:</para>

<programlisting><classname>tribool</classname> b = some_operation();
if (b) {
  // b is true
}
else if (!b) {
  // b is false
}
else {
  // b is indeterminate
}</programlisting>

  <para> <code><classname>tribool</classname></code> supports the
  3-state logic operators <code>!</code> (negation),
  <code>&amp;&amp;</code> (AND), and <code>||</code> (OR), with
  <code>bool</code> and <code><classname>tribool</classname></code>
  values. For instance:</para>

  <programlisting><classname>tribool</classname> x = some_op();
<classname>tribool</classname> 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
  }
}</programlisting>

  <para> Similarly, <code><classname>tribool</classname></code>
  supports 3-state equality comparisons via the operators
  <code>==</code> and <code>!=</code>. These operators differ from
  "normal" equality operators in C++ because they return a
  <code><classname>tribool</classname></code>, because potentially we
  might not know the result of a comparison (try to compare
  <code>true</code> and
  <code><functionname>indeterminate</functionname></code>). For
  instance:</para>

<programlisting><classname>tribool</classname> x(true);
<classname>tribool</classname> y(<functionname>indeterminate</functionname>);

assert(x == x); // okay, x == x returns true
assert(x == true); // okay, can compare <classname>tribool</classname>s and bools</programlisting>

  <para> The <code><functionname>indeterminate</functionname></code> keyword (representing the
  <functionname>indeterminate</functionname>&nbsp;<code><classname>tribool</classname></code> value)
  doubles as a function to check if the value of a
  <code><classname>tribool</classname></code> is indeterminate,
  e.g.,</para>
 
  <programlisting><classname>tribool</classname> x = try_to_do_something_tricky();
if (<functionname>indeterminate</functionname>(x)) {
  // value of x is indeterminate
}
else {
  // report success or failure of x
}</programlisting>
</section>
  
  <section>
    <title>Renaming the indeterminate state</title>
  <para> Users may introduce additional keywords for the indeterminate
  value in addition to the implementation-supplied
  <code><functionname>indeterminate</functionname></code> using the
  <code><macroname>BOOST_TRIBOOL_THIRD_STATE</macroname></code>
  macro. For instance, the following macro instantiation (at the
  global scope) will introduce the keyword <code>maybe</code> as a
  synonym for <code><functionname>indeterminate</functionname></code>
  (also residing in the <code>boost</code> namespace):</para>
  <programlisting><macroname>BOOST_TRIBOOL_THIRD_STATE</macroname>(maybe)
<classname>tribool</classname> x = maybe;
if (maybe(x)) { /* ... */ }</programlisting>
  </section>

  <section>
    <title><code>tribool</code> input/output</title>
  <para><code><classname>tribool</classname></code> objects may be
  read from and written to streams by including the
  <headername>boost/logic/tribool_io.hpp</headername> header in a
  manner very similar to <code>bool</code> values. When the
  <code>boolalpha</code> flag is not set on the input/output stream,
  the integral values 0, 1, and 2 correspond to <code>tribool</code>
  values <code>false</code>, <code>true</code>, and
  <code>indeterminate</code>, respectively. When
  <code>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:</para>
<programlisting><classname>tribool</classname> 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"</programlisting>

  <para><code><classname>tribool</classname></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><classname>std::numpunct</classname></code> facet, and the
  string naming the indeterminate type is contained in the
  <code><classname>indeterminate_name</classname></code> facet. To
  replace the name of the indeterminate state, you need to imbue your
  stream with a local containing a
  <code><classname>indeterminate_name</classname></code> facet, e.g.:</para>

  <programlisting><macroname>BOOST_TRIBOOL_THIRD_STATE</macroname>(maybe)
locale global;
locale test_locale(global, new <classname>indeterminate_name</classname>&lt;char&gt;("maybe"));
cout.imbue(test_locale);
<classname>tribool</classname> x(maybe);
cout &lt;&lt; boolalpha &lt;&lt; x &lt;&lt; endl; // Prints "maybe"</programlisting>

  <para>If you C++ standard library implementation does not support
  locales, <code>tribool</code> input/output will still work, but you
  will be unable to customize the strings printed/parsed when
  <code>boolalpha</code> is set.</para>
  </section>

  </section>

  <xi:include href="reference.boostbook"/>

  <testsuite id="tribool.tests">
    <run-test filename="tribool_test.cpp">
      <purpose><para>Test all features of the
      <code><classname>boost::logic::tribool</classname></code>
      class.</para></purpose>
    </run-test>

    <run-test filename="tribool_rename_test.cpp">
      <purpose><para>Test the use of the
      <code><macroname>BOOST_TRIBOOL_THIRD_STATE</macroname></code>
      macro.</para></purpose>
    </run-test>

    <run-test filename="tribool_io_test.cpp">
      <purpose><para>Test tribool input/output.</para></purpose>
    </run-test>
  </testsuite>
</library>