what does (template) rebind do?

The _Alloc template is used to obtain objects of some type. The container may have an internal need to allocate objects of a different type. For example, when you have a std::list<T, A>, the allocator A is meant to allocate objects of type T but the std::list<T, A> actually needs to allocate objects of some node type. Calling the node type _Ty, the std::list<T, A> needs to get hold of an allocator for _Ty objects which is using the allocation mechanism provided by A. Using

typename _A::template rebind<_Ty>::other

specifies the corresponding type. Now, there are a few syntactic annoyances in this declaration:

1. Since rebind is a member template of _A and _A is a template argument, the rebind becomes a dependent name. To indicate that a dependent name is a template, it needs to be prefixed by template. Without the template keyword the < would be considered to be the less-than operator.
2. The name other also depends on a template argument, i.e., it is also a dependent name. To indicate that a dependent name is a type, the typename keyword is needed.