title: How to initialize a bitset type all bits to 1
body: <p>I know set() function for a already constructed bitset object, but I need a constructed bitset which all bits are 1. The situation is a default function parameter.
  for example:</p>
  
  <pre><code>void bar(std::bitset&lt;100&gt; flags = X) {
  }
  </code></pre>
  
  <p>what X should be, -1 may works for first 64 bits, but not all.</p>