1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Tutorial</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../tribool.html" title="Chapter 23. Boost.Tribool"> 10<link rel="prev" href="../tribool.html" title="Chapter 23. Boost.Tribool"> 11<link rel="next" href="reference.html" title="Reference"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<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> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="tribool.tutorial"></a>Tutorial</h2></div></div></div> 29<div class="toc"><dl class="toc"> 30<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.2">Basic usage</a></span></dt> 31<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.3">Renaming the indeterminate state</a></span></dt> 32<dt><span class="section"><a href="tutorial.html#id-1.3.24.4.4"><code class="computeroutput">tribool</code> input/output</a></span></dt> 33</dl></div> 34<div class="section"> 35<div class="titlepage"><div><div><h3 class="title"> 36<a name="id-1.3.24.4.2"></a>Basic usage</h3></div></div></div> 37<p> The <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class acts 38 like the built-in <code class="computeroutput">bool</code> type, but for 3-state boolean 39 logic. The three states are <code class="computeroutput">true</code>, <code class="computeroutput">false</code>, 40 and <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>, where 41 the first two states are equivalent to those of the C++ 42 <code class="computeroutput">bool</code> type and the last state represents an unknown 43 boolean value (that may be <code class="computeroutput">true</code> or 44 <code class="computeroutput">false</code>, we don't know).</p> 45<p> The <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> class 46 supports conversion from <code class="computeroutput">bool</code> values and literals 47 along with its own 48 <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> 49 keyword:</p> 50<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b(true); 51b = false; 52b = <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>; 53<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b2(b);</pre> 54<p> <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports 55 conversions to <code class="computeroutput">bool</code> for use in conditional 56 statements. The conversion to <code class="computeroutput">bool</code> will be 57 <code class="computeroutput">true</code> when the value of the 58 <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is always true, and 59 <code class="computeroutput">false</code> otherwise. Consequently, the following idiom 60 may be used to determine which of the three states a 61 <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> currently 62 holds:</p> 63<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> b = some_operation(); 64if (b) { 65 // b is true 66} 67else if (!b) { 68 // b is false 69} 70else { 71 // b is indeterminate 72}</pre> 73<p> <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> supports the 74 3-state logic operators <code class="computeroutput">!</code> (negation), 75 <code class="computeroutput">&&</code> (AND), and <code class="computeroutput">||</code> (OR), with 76 <code class="computeroutput">bool</code> and <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> 77 values. For instance:</p> 78<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = some_op(); 79<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> y = some_other_op(); 80if (x && y) { 81 // both x and y are true 82} 83else if (!(x && y)) { 84 // either x or y is false 85} 86else { 87 // neither x nor y is false, but we don't know that both are true 88 89 if (x || y) { 90 // either x or y is true 91 } 92}</pre> 93<p> Similarly, <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> 94 supports 3-state equality comparisons via the operators 95 <code class="computeroutput">==</code> and <code class="computeroutput">!=</code>. These operators differ from 96 "normal" equality operators in C++ because they return a 97 <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code>, because potentially we 98 might not know the result of a comparison (try to compare 99 <code class="computeroutput">true</code> and 100 <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>). For 101 instance:</p> 102<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(true); 103<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>); 104 105assert(x == x); // okay, x == x returns true 106assert(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> 107<p> The <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> keyword (representing the 108 <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) 109 doubles as a function to check if the value of a 110 <code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> is indeterminate, 111 e.g.,</p> 112<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(); 113if (<code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code>(x)) { 114 // value of x is indeterminate 115} 116else { 117 // report success or failure of x 118}</pre> 119<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 120 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 121 be used in compile time expressions:</p> 122<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)); 123<code class="computeroutput">static_assert</code>(x, "Must be true!"); 124</pre> 125<div class="note"><table border="0" summary="Note"> 126<tr> 127<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> 128<th align="left">Note</th> 129</tr> 130<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> 131</table></div> 132</div> 133<div class="section"> 134<div class="titlepage"><div><div><h3 class="title"> 135<a name="id-1.3.24.4.3"></a>Renaming the indeterminate state</h3></div></div></div> 136<p> Users may introduce additional keywords for the indeterminate 137 value in addition to the implementation-supplied 138 <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> using the 139 <code class="computeroutput"><a class="link" href="../BOOST_TRIBOOL_THIRD_STATE.html" title="Macro BOOST_TRIBOOL_THIRD_STATE">BOOST_TRIBOOL_THIRD_STATE</a></code> 140 macro. For instance, the following macro instantiation (at the 141 global scope) will introduce the keyword <code class="computeroutput">maybe</code> as a 142 synonym for <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate.html" title="Function indeterminate">indeterminate</a></code> 143 (also residing in the <code class="computeroutput">boost</code> namespace):</p> 144<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) 145<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x = maybe; 146if (maybe(x)) { /* ... */ }</pre> 147</div> 148<div class="section"> 149<div class="titlepage"><div><div><h3 class="title"> 150<a name="id-1.3.24.4.4"></a><code class="computeroutput">tribool</code> input/output</h3></div></div></div> 151<p><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> objects may be 152 read from and written to streams by including the 153 <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 154 manner very similar to <code class="computeroutput">bool</code> values. When the 155 <code class="computeroutput">boolalpha</code> flag is not set on the input/output stream, 156 the integral values 0, 1, and 2 correspond to <code class="computeroutput">tribool</code> 157 values <code class="computeroutput">false</code>, <code class="computeroutput">true</code>, and 158 <code class="computeroutput">indeterminate</code>, respectively. When 159 <code class="computeroutput">boolalpha</code> is set on the stream, arbitrary strings can 160 be used to represent the three values, the default being "false", 161 "true", and "indeterminate". For instance:</p> 162<pre class="programlisting"><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x; 163cin >> x; // Type "0", "1", or "2" to get false, true, or indeterminate 164cout << boolalpha << x; // Produces "false", "true", or "indeterminate"</pre> 165<p><code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> input and output 166 is sensitive to the stream's current locale. The strings associated 167 with false and true values are contained in the standard 168 <code class="computeroutput">std::numpunct</code> facet, and the 169 string naming the indeterminate type is contained in the 170 <code class="computeroutput"><a class="link" href="../boost/logic/indeterminate_name.html" title="Class template indeterminate_name">indeterminate_name</a></code> facet. To 171 replace the name of the indeterminate state, you need to imbue your 172 stream with a local containing a 173 <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> 174<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) 175locale global; 176locale 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")); 177cout.imbue(test_locale); 178<code class="computeroutput"><a class="link" href="../boost/logic/tribool.html" title="Class tribool">tribool</a></code> x(maybe); 179cout << boolalpha << x << endl; // Prints "maybe"</pre> 180<p>If you C++ standard library implementation does not support 181 locales, <code class="computeroutput">tribool</code> input/output will still work, but you 182 will be unable to customize the strings printed/parsed when 183 <code class="computeroutput">boolalpha</code> is set.</p> 184</div> 185</div> 186<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 187<td align="left"></td> 188<td align="right"><div class="copyright-footer">Copyright © 2002-2004 Douglas Gregor<p>Use, modification and distribution is subject to the Boost 189 Software License, Version 1.0. (See accompanying file 190 <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> 191</div></td> 192</tr></table> 193<hr> 194<div class="spirit-nav"> 195<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> 196</div> 197</body> 198</html> 199
