 <p>It simply replaces <code>x</code> with <code>1.0f</code>, so:</p> <pre><code>NSNumber *n = FBOX(1.0f); </code></pre> <p>becomes:</p> <pre><code>NSNumber *n = [NSNumber numberWithFloat:1.0f]; </code></pre> <p>this is then presented to the compiler.</p>