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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_convert.alternative_interface"></a><a class="link" href="alternative_interface.html" title="Alternative Interface">Alternative Interface</a>
</h2></div></div></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="bold"><strong><span class="emphasis"><em>"It's not a question of happiness, it's
a requirement. Consider the alternative” Doug Horton</em></span></strong></span>
</p></blockquote></div>
<p>
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>
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:
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">TypeOut</span><span class="special">&gt;</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">&amp;,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&amp;);</span>
</pre>
<p>
The signature is <span class="emphasis"><em>functionally-complete</em></span> and routinely elided
during compilation. Still, the following alternative (and arguably more traditional)
interface might be potentially more suitable for certain deployment scenarios
(or due to personal preferences):
</p>
<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">&amp;,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&amp;,</span> <span class="identifier">TypeOut</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">fallback_value</span><span class="special">);</span>
<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">&amp;,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&amp;,</span> <span class="identifier">Functor</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">fallback_func</span><span class="special">);</span>
<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">&amp;,</span> <span class="identifier">Converter</span> <span class="keyword">const</span><span class="special">&amp;,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">throw_on_failure</span><span class="special">);</span>
</pre>
<p>
The interface still provides unambiguous behavior and readability, full support
for various program flows and various degrees of conversion-failure detection
and processing. It can be deployed in a similar fashion as follows:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">fallback_func</span>
<span class="special">{</span>
<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>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// Error-processing behavior are specified unambiguously and uniformly.</span>
<span class="comment">// a) i1: Returns the provided fallback value;</span>
<span class="comment">// b) i2: Calls the provided failure-processing function;</span>
<span class="comment">// c) i3: Throws an exception.</span>
<span class="keyword">int</span> <span class="identifier">i1</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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>
<span class="keyword">int</span> <span class="identifier">i2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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>
<span class="keyword">try</span>
<span class="special">{</span>
<span class="comment">// Throwing behavior specified explicitly rather than implied.</span>
<span class="keyword">int</span> <span class="identifier">i3</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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>
<span class="special">}</span>
<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">&amp;)</span>
<span class="special">{</span>
<span class="comment">// Handle failed conversion.</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
Still, the described interfaces are convenience wrappers around the main interface
which provides the described behavior with:
</p>
<p>
</p>
<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">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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>
<span class="keyword">int</span> <span class="identifier">m2</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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>
<span class="keyword">int</span> <span class="identifier">m3</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</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>
</pre>
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