When to use C++ forward_list

Question

I am kind of new to C++, and reading the book \"The C++ Programming Language (4th edition)\". When reading chapter of \"STL Containers\", the book has a introduction to forward_

Answer 1

Here is the node structure used in the std::list and std::forward_list:

struct _List_node      // list node
{   
  _Voidptr _Next;      // successor node, or first element if head
  _Voidptr _Prev;      // predecessor node, or last element if head
  _Value_type _Myval;  // the stored value, unused if head
};

struct _Flist_node     // forward_list node
{   
  _Voidptr _Next;      // successor node
  _Value_type _Myval;  // the stored value
};

Space calculation:

sizeof(_List_node)  = 2 * sizeof(_Voidptr) + sizeof(_Value_type)
sizeof(_Flist_node) = 1 * sizeof(_Voidptr) + sizeof(_Value_type)

if sizeof(_Value_type) is same or approximately equal to sizeof(_Voidptr) then we have:

sizeof(_List_node)  ~ 3 * sizeof(_Voidptr)
sizeof(_Flist_node) ~ 2 * sizeof(_Voidptr)

If sizeof(_Value_type) >> sizeof(_Voidptr) then the savings is negligible. But for storing smaller objects using forward_list has ~1.5x savings in space (provided you don't need a reverse iterator.)