title: How to define an implicit conversion from double?
body: <p>I have defined a class <code>Complex</code> which overloads the <code>+</code> operator:</p>
  
  <pre><code>Complex operator+(Complex const&amp; x, Complex const&amp; y);
  </code></pre>
  
  <p>I want to define an implicit conversion from <code>double</code> to <code>Complex</code>, such that, for example, if I write <code>c + d</code>, where <code>c</code> is a <code>Complex</code> and <code>d</code> a <code>double</code>, it will call my overloaded <code>+</code> that I defined above and return a <code>Complex</code>. How can I do this?</p>