text: #ifndef MUDUO_BASE_COPYABLE_H
  #define MUDUO_BASE_COPYABLE_H
  
  namespace muduo
  {
  
  /// A tag class emphasises the objects are copyable.
  /// The empty base class optimization applies.
  /// Any derived class of copyable should be a value type.
  class copyable
  {
   protected:
    copyable() = default;
    ~copyable() = default;
  };
  
  };
  
  #endif  // MUDUO_BASE_COPYABLE_H
  