multiple_inheritance_STL.txt

These are the only examples of MI that I see as of 4/19/20 in /usr/include/c++/9.

/usr/include/c++/9/std_function.h:
  /**
   *  @brief Primary class template for std::function.
   *  @ingroup functors
   *
   *  Polymorphic function wrapper.
   */
  template<typename _Res, typename... _ArgTypes>
    class function<_Res(_ArgTypes...)>
    : public _Maybe_unary_or_binary_function<_Res, _ArgTypes...>,
      private _Function_base

In /usr/include/c++/9/unique_ptr.h:
  /// std::hash specialization for unique_ptr.
  template<typename _Tp, typename _Dp>
    struct hash<unique_ptr<_Tp, _Dp>>
    : public __hash_base<size_t, unique_ptr<_Tp, _Dp>>,
    private __poison_hash<typename unique_ptr<_Tp, _Dp>::pointer>

Easy to see why, for example, stringstream objects might end up with
multiple associated ios_base objects if the basic streams did not
inherit from basic_ios with "virtual public."
See diagram at http://www.cplusplus.com/reference/sstream/stringstream/.
$ grep -rI "virtual public" /usr/include/c++/9
/usr/include/c++/9/istream:    class basic_istream : virtual public basic_ios<_CharT, _Traits>
/usr/include/c++/9/ostream:    class basic_ostream : virtual public basic_ios<_CharT, _Traits>