Optional value returning nil with ?? operator : <p>I'm trying to convert an optional string to a Double, but when I unwrap my value is not taken:</p>

<pre><code>let currentSwimDistanceTravelled = defaults.string(forKey: "SwimTotal") ?? "0.0"
</code></pre>

<p>After this code is run currentSwimDistanceTravelled is assigned a String value of "" as opposed to "0.0".</p>

<p>Sorry if I've missed something basic here, but I thought ?? was the best way to unwrap an optional.</p>
