<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Tutorial</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../tribool.html" title="Chapter 24. Boost.Tribool"> <link rel="prev" href="../tribool.html" title="Chapter 24. Boost.Tribool"> <link rel="next" href="reference.html" title="Reference"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> <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#id2761741">Basic usage</a></span></dt> <dt><span class="section"><a href="tutorial.html#id2762214">Renaming the indeterminate state</a></span></dt> <dt><span class="section"><a href="tutorial.html#id2762301"><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="id2761741"></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="id2762214"></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="id2762301"></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> </div></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> </body> </html>