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