title: How properly instantiate static field tat is another class object
body: <p>If I have in my .h file</p>
  
  <pre><code>class A
  {
   ...
  protected:
  static std::mutex someMutex;
  }
  </code></pre>
  
  <p>What is proper way to instantiate it in the cpp file?
  Just to write </p>
  
  <pre><code>std::mutex A::someMutex;
  </code></pre>
  
  <p>? It works, but just such syntax where I basically declare a field twice feels a bit confusing, so I decided to doublecheck asking here, also maybe there are some other(better?) ways?</p>