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| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="tribool.tutorial"></a>Tutorial</h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="tutorial.html#id2751889">Basic usage</a></span></dt> |
| <dt><span class="section"><a href="tutorial.html#id2752362">Renaming the indeterminate state</a></span></dt> |
| <dt><span class="section"><a href="tutorial.html#id2752448"><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="id2751889"></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">&&</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 && y) { |
| // both x and y are true |
| } |
| else if (!(x && 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> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="id2752362"></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="id2752448"></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 <boost/logic/tribool_io.hpp>">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 >> x; // Type "0", "1", or "2" to get false, true, or indeterminate |
| cout << boolalpha << 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><char>("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 << boolalpha << x << 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> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <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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