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<title>Getting Started</title>
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<link rel="home" href="../index.html" title="Chapter 1. The Boost Convert Library 2.0">
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<link rel="up" href="../index.html" title="Chapter 1. The Boost Convert Library 2.0">
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<link rel="prev" href="convert_design.html" title="Design Notes">
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<link rel="next" href="convert_getting_serious.html" title="Getting Serious">
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="the_boost_convert_library.convert_started"></a><a class="link" href="convert_started.html" title="Getting Started">Getting Started</a>
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</h2></div></div></div>
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<p>
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In its simplest form the conversion functionality might be deployed as follows:
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</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">api</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</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">sstream_converter</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">;</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">cstringstream_converter</span> <span class="identifier">cnv</span><span class="special">;</span> <span class="comment">// stringstream-based char converter</span>
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<span class="keyword">int</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">boost</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> <span class="comment">// Throws if the conversion fails</span>
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<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">from</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> <span class="comment">// Throws if the conversion fails</span>
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<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">from</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="number">1</span><span class="special">);</span> <span class="comment">// Returns -1 if the conversion fails</span>
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</pre>
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<p>
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These calls can be translated to plain English as "<span class="emphasis"><em>Convert a
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string to int</em></span>" for <span class="emphasis"><em>i1</em></span> and <span class="emphasis"><em>i2</em></span>
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and "<span class="emphasis"><em>Convert a string to int and return -1 if the conversion
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fails</em></span>" for <span class="emphasis"><em>i3</em></span>.
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</p>
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<p>
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The <span class="emphasis"><em>i1</em></span> and <span class="emphasis"><em>i2</em></span> deployments look sufficiently
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close and, in fact, are identical behaviorally. Namely, if the requested conversion
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fails, an exception is thrown. It is due to the fact that the user request
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-- "<span class="emphasis"><em>Convert a string to int</em></span>" -- lacks the specification
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for the failure use-case. Consequently, <span class="emphasis"><em>boost::lexical_cast</em></span>
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and <span class="emphasis"><em>boost::convert</em></span> take the liberty of handling that not-mentioned
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use-case in the standard way.
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</p>
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<p>
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The second specification -- "<span class="emphasis"><em>Convert a string to int and return
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-1 if the conversion fails</em></span>" -- is complete and processed "as
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ordered" with the provided fallback value returned if the requested conversion
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fails.
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</p>
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<p>
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The described (very basic) interface might be sufficient for a variety of conversion
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deployments. For example:
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</p>
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<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cstringstream_converter</span> <span class="identifier">ccnv</span><span class="special">;</span> <span class="comment">// stringstream-based char converter</span>
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<span class="keyword">int</span> <span class="identifier">num_threads</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">from</span><span class="special">(</span><span class="identifier">str1</span><span class="special">,</span> <span class="identifier">ccnv</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">INT_MAX</span><span class="special">);</span> <span class="comment">// Read as hex</span>
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<span class="keyword">int</span> <span class="identifier">num_windows</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">from</span><span class="special">(</span><span class="identifier">str2</span><span class="special">,</span> <span class="identifier">ccnv</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">INT_MAX</span><span class="special">);</span> <span class="comment">// Read as decimal</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">num_threads</span> <span class="special">==</span> <span class="identifier">INT_MAX</span><span class="special">)</span> <span class="identifier">log</span><span class="special">(</span><span class="string">"bad num_threads"</span><span class="special">),</span> <span class="identifier">num_threads</span> <span class="special">=</span> <span class="identifier">default_num_threads</span><span class="special">;</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">num_windows</span> <span class="special">==</span> <span class="identifier">INT_MAX</span><span class="special">)</span> <span class="identifier">log</span><span class="special">(</span><span class="string">"bad num_windows"</span><span class="special">),</span> <span class="identifier">num_windows</span> <span class="special">=</span> <span class="identifier">default_num_windows</span><span class="special">;</span>
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<span class="special">...</span> <span class="identifier">proceed</span>
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</pre>
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<p>
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Or even shorter, if we do not care logging:
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</p>
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<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">num_threads</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">from</span><span class="special">(</span><span class="identifier">str1</span><span class="special">,</span> <span class="identifier">ccnv</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_num_threads</span><span class="special">);</span> <span class="comment">// Read as hex</span>
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<span class="keyword">int</span> <span class="identifier">num_windows</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">from</span><span class="special">(</span><span class="identifier">str2</span><span class="special">,</span> <span class="identifier">ccnv</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_num_windows</span><span class="special">);</span> <span class="comment">// Read as decimal</span>
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<span class="special">...</span> <span class="identifier">proceed</span>
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</pre>
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<p>
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So far the deployment of <span class="emphasis"><em>boost::convert</em></span> seems more compact
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and natural (your mileage may vary) and might be potentially more efficient
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compared to <span class="emphasis"><em>boost::lexical_cast</em></span> deployment which achieves
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a similar (excluding formatting) result with something like
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</p>
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<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">num_threads</span><span class="special">;</span>
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<span class="keyword">try</span>
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<span class="special">{</span>
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<span class="identifier">num_threads</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>
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<span class="special">}</span>
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<span class="keyword">catch</span> <span class="special">(...)</span>
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<span class="special">{</span>
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<span class="identifier">log</span><span class="special">(</span><span class="string">"bad num_threads"</span><span class="special">);</span>
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<span class="identifier">num_threads</span> <span class="special">=</span> <span class="identifier">default_num_threads</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<p>
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This is by-design <span class="emphasis"><em>boost::lexical_cast</em></span>'s only behavior.
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Simple, straigtforward and comprehensible. Unfortunately, it makes quite a
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few legitimate process flows difficult/awkward to implement. That is the niche
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that <span class="emphasis"><em>boost::convert</em></span> is trying to fill.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2009-2014 Vladimir Batov<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></td>
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</tr></table>
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<hr>
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