title: Initialisation of static class member. Why constexpr?
body: <p>when I want to have a static pointer as a member of a class I need <code>constexpr</code>for the initialisation with <code>nullptr</code>.</p>
  
  <pre><code>class Application {
      private:
          constexpr static Application* app = nullptr;
  }
  </code></pre>
  
  <p>Can someone explain me why I need to do that? I cannot find the exact reason why it`s necessary that the static variable has to exist at compile time.</p>