More C++ Idioms/Empty Base Optimization

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 template <class Base, class Member>
   struct BaseOpt : Base {
     Member m;
     BaseOpt(Base const& b, Member const& mem) 
       : Base(b), m(mem) { }
   };

Using this template, our list<> declaration appears as in Listing 6.

Listing 6: The Best Way to Eliminate Bloat

 template <class T, class Alloc = allocator<T> >
   class list {
     . . .
     struct Node { . . . };
     BaseOpt<Alloc,Node*> head_;
     
    public:
     explicit list(Alloc const& a = Alloc())
       : head_(a,0) { . . . }
     . . .
   };

read here http://www.cantrip.org/emptyopt.html