You're either gonna write #include "thing.h" or class Thing; Either way, as a pr...

cgrealy · on Sept 22, 2020

You are repeating yourself if you write

  class Thing;

in every file that uses a Thing. And as I said, that's generally fine.

The issue becomes when the declaration of Thing includes namespaces, template params, etc. Rewriting that is tedious and error prone.

Look at something like std::string. That's actually

  namespace std {
  template< class T > struct allocator;
  template<class charT> struct char_traits;
  template<> struct char_traits<char>;
  template<class CharT, class Traits = char_traits<CharT>,
           class Allocator = allocator<CharT>>
    class basic_string;
  using string    = basic_string<char>;
  }

No-one is forward declaring that.