title: Ostream << and operator -= for classes?
body: <p>I have 2 classes <code>S</code> and <code>M</code>. When I try to use</p>
  
  <pre><code>cout &lt;&lt; s3 -= m2;
  </code></pre>
  
  <p>I get an error that sates:</p>
  
  <blockquote>
    <p>no operator "-=" matches these operands  operand types are: std::ostream -= </p>
  </blockquote>
  
  <pre><code>class S
  { 
  public:
      S&amp; operator-=(M&amp; m)
      {
          //my code
          return *this;
      }
  }
  </code></pre>
  
  <p>I tried with 3 parameters, including <code>ostream</code>, but <code>-=</code> has only 2.
  How can I fix this?</p>