1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Alternative Interface</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="../index.html" title="Chapter 1. Boost.Convert 2.0"> 9<link rel="prev" href="default_converter.html" title="Default Converter"> 10<link rel="next" href="converters.html" title="Converters"> 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="default_converter.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="converters.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="boost_convert.alternative_interface"></a><a class="link" href="alternative_interface.html" title="Alternative Interface">Alternative Interface</a> 28</h2></div></div></div> 29<div class="blockquote"><blockquote class="blockquote"><p> 30 <span class="bold"><strong><span class="emphasis"><em>"It's not a question of happiness, it's 31 a requirement. Consider the alternative” Doug Horton</em></span></strong></span> 32 </p></blockquote></div> 33<p> 34 As it was indicated previously <a href="../../../../../libs/optional/index.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code></a> 35 is the actual type returned by the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">()</span></code> main interface: 36 </p> 37<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">TypeOut</span><span class="special">></span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">(</span><span class="identifier">TypeIn</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&);</span> 38</pre> 39<p> 40 The signature is <span class="emphasis"><em>functionally-complete</em></span> and routinely elided 41 during compilation. Still, the following alternative (and arguably more traditional) 42 interface might be potentially more suitable for certain deployment scenarios 43 (or due to personal preferences): 44 </p> 45<pre class="programlisting"><span class="identifier">TypeOut</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">TypeIn</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">TypeOut</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fallback_value</span><span class="special">);</span> 46<span class="identifier">TypeOut</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">TypeIn</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">Functor</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fallback_func</span><span class="special">);</span> 47<span class="identifier">TypeOut</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">TypeIn</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">throw_on_failure</span><span class="special">);</span> 48</pre> 49<p> 50 The interface still provides unambiguous behavior and readability, full support 51 for various program flows and various degrees of conversion-failure detection 52 and processing. It can be deployed in a similar fashion as follows: 53 </p> 54<p> 55</p> 56<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">fallback_func</span> 57<span class="special">{</span> 58 <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="identifier">log</span><span class="special">(</span><span class="string">"Failed to convert"</span><span class="special">);</span> <span class="keyword">return</span> <span class="number">42</span><span class="special">;</span> <span class="special">}</span> 59<span class="special">};</span> 60</pre> 61<p> 62 </p> 63<p> 64</p> 65<pre class="programlisting"><span class="comment">// Error-processing behavior are specified unambiguously and uniformly.</span> 66<span class="comment">// a) i1: Returns the provided fallback value;</span> 67<span class="comment">// b) i2: Calls the provided failure-processing function;</span> 68<span class="comment">// c) i3: Throws an exception.</span> 69 70<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">str</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">,</span> <span class="identifier">fallback_value</span><span class="special">);</span> 71<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">str</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">,</span> <span class="identifier">fallback_func</span><span class="special">());</span> 72 73<span class="keyword">try</span> 74<span class="special">{</span> 75 <span class="comment">// Throwing behavior specified explicitly rather than implied.</span> 76 <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="identifier">str</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">throw_on_failure</span><span class="special">);</span> 77<span class="special">}</span> 78<span class="keyword">catch</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span> <span class="keyword">const</span><span class="special">&)</span> 79<span class="special">{</span> 80 <span class="comment">// Handle failed conversion.</span> 81<span class="special">}</span> 82</pre> 83<p> 84 </p> 85<p> 86 Still, the described interfaces are convenience wrappers around the main interface 87 which provides the described behavior with: 88 </p> 89<p> 90</p> 91<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">m1</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">str</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="identifier">fallback_value</span><span class="special">);</span> 92<span class="keyword">int</span> <span class="identifier">m2</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">str</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">fallback_func</span><span class="special">());</span> 93<span class="keyword">int</span> <span class="identifier">m3</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">str</span><span class="special">,</span> <span class="identifier">cnv</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span> 94</pre> 95<p> 96 </p> 97</div> 98<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 99<td align="left"></td> 100<td align="right"><div class="copyright-footer">Copyright © 2009-2016 Vladimir Batov<p> 101 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>. 102 </p> 103</div></td> 104</tr></table> 105<hr> 106<div class="spirit-nav"> 107<a accesskey="p" href="default_converter.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="converters.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 108</div> 109</body> 110</html> 111
