There is an obscure pitfall of making a class noncopyable by making the copy constructor private.

Consider the following class:

  class X { 
   private:
     X& (const X&); // not defined
   public:
     X bar() {
        X x = ......;
        return x;
     }
   };

Due to programmer error, an instance of X is returned by value despite the class being noncopyable. The copy constructor being private doesn't help as bar() is a method of X. This will compile and link, despite the copy constructor being undefined. I will leave the reason for this as an exercise for the reader.