Question

I am reading STL source code right now.
Though I understand the meat in what I am reading in stl_list.h, I want to fully understand the following snippet (mainly related to the template syntax, I think).

template

class _List_base {
  ...
  typedef typename _Alloc::template rebind<_List_node<_Tp> >::other _Node_Alloc_type; //(1).

  ...
  typedef _Alloc allocator_type;
  get_allocator() const
  { return allocator_type(*static_cast<
                          const _Node_Alloc_type*>(&this->_M_impl)); }  // (2)
  ...
};

Can someone explain why we need a “template” following _Alloc in line (1)? (and giving a full explanation of this line?)

Can someone explain why we can cast _Node_Alloc_type to _Alloc in line (2)?

Answer 1

The template keyword is needed to identify the name rebind as a class template. Without it, rebind could be considered a variable or a constant (in this case a type due to the typename keyword) and the following < could be interpreted as a less-than operator.

This is somewhat similar to the typename keyword (which is of course necessary to identify other as a type).

Every allocator is required to provide a meta-function (i.e. a class template) called rebind that returns the same allocator but for a different type. In other words,

Alloc<T>::rebind<U>::other

names the same type as

Alloc<U>

The second part of your question is difficult to answer without more context. What is the type of _M_impl? How is that type defined?