1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Basic Conversion-Failure Detection</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Boost.Convert 2.0"> 8<link rel="up" href="../getting_started.html" title="Getting Started"> 9<link rel="prev" href="flexibility_and_adaptability_to_change.html" title="Flexibility and Adaptability to Change"> 10<link rel="next" href="../error_detection.html" title="Better Error Detection"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="flexibility_and_adaptability_to_change.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../getting_started.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="../error_detection.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_convert.getting_started.basic_conversion_failure_detection"></a><a class="link" href="basic_conversion_failure_detection.html" title="Basic Conversion-Failure Detection">Basic 28 Conversion-Failure Detection</a> 29</h3></div></div></div> 30<p> 31</p> 32<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span> 33<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">;</span> 34<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">;</span> 35</pre> 36<p> 37 </p> 38<p> 39</p> 40<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">cstream</span> <span class="identifier">cnv</span><span class="special">;</span> 41 42<span class="keyword">int</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"123"</span><span class="special">);</span> <span class="comment">// Throws when conversion fails.</span> 43<span class="keyword">int</span> <span class="identifier">i2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"123"</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// Throws when conversion fails.</span> 44<span class="keyword">int</span> <span class="identifier">i3</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"uhm"</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(-</span><span class="number">1</span><span class="special">);</span> <span class="comment">// Returns -1 when conversion fails.</span> 45 46<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">i1</span> <span class="special">==</span> <span class="number">123</span><span class="special">);</span> 47<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">i2</span> <span class="special">==</span> <span class="number">123</span><span class="special">);</span> 48<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">i3</span> <span class="special">==</span> <span class="special">-</span><span class="number">1</span><span class="special">);</span> 49</pre> 50<p> 51 </p> 52<p> 53 The above is translated to English as 54 </p> 55<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 56<li class="listitem"> 57 "<span class="emphasis"><em>convert a string to int</em></span>" for <code class="computeroutput"><span class="identifier">i1</span></code> and <code class="computeroutput"><span class="identifier">i2</span></code> 58 and 59 </li> 60<li class="listitem"> 61 "<span class="emphasis"><em>convert a string to int and return -1 if the conversion 62 fails</em></span>" for <code class="computeroutput"><span class="identifier">i3</span></code>. 63 </li> 64</ul></div> 65<p> 66 The <code class="computeroutput"><span class="identifier">i1</span></code> and <code class="computeroutput"><span class="identifier">i2</span></code> deployments look sufficiently close 67 and behave identically. Namely, with the user instructions silent about the 68 conversion failure, those are treated as "exceptional" and throw. 69 </p> 70<p> 71 The <code class="computeroutput"><span class="identifier">i3</span></code> specification, on 72 the other hand, is explicit about conversion failures. The supplied fallback 73 value is returned if the requested conversion fails. 74 </p> 75<p> 76 That basic error detection and processing might be sufficient for a variety 77 of conversion deployments. For example: 78 </p> 79<p> 80</p> 81<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">s1</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">hex</span><span class="special">)).</span><span class="identifier">value_or</span><span class="special">(-</span><span class="number">1</span><span class="special">);</span> <span class="comment">// Read as hex</span> 82<span class="keyword">int</span> <span class="identifier">i2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">s2</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">dec</span><span class="special">)).</span><span class="identifier">value_or</span><span class="special">(-</span><span class="number">1</span><span class="special">);</span> <span class="comment">// Read as decimal</span> 83 84<span class="keyword">if</span> <span class="special">(</span><span class="identifier">i1</span> <span class="special">==</span> <span class="special">-</span><span class="number">1</span><span class="special">)</span> <span class="identifier">log</span><span class="special">(</span><span class="string">"bad i1"</span><span class="special">),</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">default_i1</span><span class="special">;</span> <span class="comment">// Log failure. Proceed with the default</span> 85<span class="keyword">if</span> <span class="special">(</span><span class="identifier">i2</span> <span class="special">==</span> <span class="special">-</span><span class="number">1</span><span class="special">)</span> <span class="identifier">log</span><span class="special">(</span><span class="string">"bad i2"</span><span class="special">),</span> <span class="identifier">i2</span> <span class="special">=</span> <span class="identifier">default_i2</span><span class="special">;</span> <span class="comment">// Log failure. Proceed with the default</span> 86 87<span class="comment">// ... proceed</span> 88</pre> 89<p> 90 </p> 91<p> 92 Or 93 </p> 94<p> 95</p> 96<pre class="programlisting"><span class="keyword">int</span> 97<span class="identifier">fallback_fun</span><span class="special">(</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">msg</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">fallback_value</span><span class="special">)</span> 98<span class="special">{</span> 99 <span class="comment">// The principal advantage of a fallback_func over a fallback_value</span> 100 <span class="comment">// is that the former is only called when the conversion request fails.</span> 101 <span class="comment">// Consequently, the returned fallback_value is only calculated (which</span> 102 <span class="comment">// potentially might be expensive) when it is absolutely necessary.</span> 103 <span class="identifier">log</span><span class="special">(</span><span class="identifier">msg</span><span class="special">);</span> <span class="keyword">return</span> <span class="identifier">fallback_value</span><span class="special">;</span> 104<span class="special">}</span> 105</pre> 106<p> 107 </p> 108<p> 109</p> 110<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">s1</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">fallback_fun</span><span class="special">,</span> <span class="string">"bad i1"</span><span class="special">,</span> <span class="identifier">default_i1</span><span class="special">));</span> 111<span class="keyword">int</span> <span class="identifier">i2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">s2</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">fallback_fun</span><span class="special">,</span> <span class="string">"bad i2"</span><span class="special">,</span> <span class="identifier">default_i2</span><span class="special">));</span> 112<span class="comment">// ... proceed</span> 113</pre> 114<p> 115 </p> 116<p> 117 Or, if we do not care about logging conversion failures: 118 </p> 119<p> 120</p> 121<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">s1</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">hex</span><span class="special">)).</span><span class="identifier">value_or</span><span class="special">(</span><span class="identifier">default_i1</span><span class="special">);</span> <span class="comment">// If failed, proceed with the default</span> 122<span class="keyword">int</span> <span class="identifier">i2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">s2</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">dec</span><span class="special">)).</span><span class="identifier">value_or</span><span class="special">(</span><span class="identifier">default_i2</span><span class="special">);</span> <span class="comment">// If failed, proceed with the default</span> 123<span class="comment">// ... proceed</span> 124</pre> 125<p> 126 </p> 127<p> 128 So far the deployment of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span></code> 129 seems more flexible, more compact and natural (your mileage may vary) and 130 potentially more efficient compared to <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> 131 which achieves somewhat similar results with: 132 </p> 133<p> 134</p> 135<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">default_i1</span><span class="special">;</span> 136 137<span class="keyword">try</span> 138<span class="special">{</span> 139 <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">str</span><span class="special">);</span> 140<span class="special">}</span> 141<span class="keyword">catch</span> <span class="special">(...)</span> 142<span class="special">{</span> 143 <span class="identifier">log</span><span class="special">(</span><span class="string">"bad i1"</span><span class="special">);</span> 144<span class="special">}</span> 145</pre> 146<p> 147 </p> 148<p> 149 By design, this is <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>'s 150 only behavior -- straightforward and comprehensible, but limited. It makes 151 quite a few legitimate process/program flows difficult and awkward to implement. 152 <span class="emphasis"><em>Boost.Convert</em></span> addresses that with additional functionality, 153 flexibility and convenience. 154 </p> 155</div> 156<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 157<td align="left"></td> 158<td align="right"><div class="copyright-footer">Copyright © 2009-2016 Vladimir Batov<p> 159 Distributed under the Boost Software License, Version 1.0. See copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>. 160 </p> 161</div></td> 162</tr></table> 163<hr> 164<div class="spirit-nav"> 165<a accesskey="p" href="flexibility_and_adaptability_to_change.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../getting_started.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="../error_detection.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 166</div> 167</body> 168</html> 169
