Added func value_or_eval()

This commit is contained in:
Andrzej Krzemienski
2014-06-18 15:01:52 +02:00
parent a158b85bd6
commit d70114b3dc
14 changed files with 536 additions and 50 deletions

View File

@ -111,6 +111,20 @@
This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
Finally, you can provide a callback to be called when trying to access the
contained value fails:
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">([]()</span> <span class="special">-&gt;</span> <span class="keyword">int</span> <span class="special">{</span>
<span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"could not convert; using -1 instead"</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
<span class="special">});</span>
</pre>
<p>
This will call the provided callback and return whatever the callback returns.
The callback can have side effects: they will only be observed when the optional
object does not contain a value.
</p>
<p>
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
can be implemented.
</p>