Why is allocator::rebind necessary when we have template template parameters?

Question

Every allocator class must have an interface similar to the following:

template
class allocator
{
    ...
    template
    s         


        

Answer 1

Suppose you want to write a function taking all sorts of vectors.

Then it is much more convenient being able to write

template 
void f (std::vector  vec) {
   // ...
}

than having to write

template  class A>
void f (std::vector  vec) {
   // ...
}

In most of the cases, such a function does not care about the allocator anyway.

Further note that allocators are not required to be a template. You could write separate classes for particular types that need to be allocated.

An even more convenient way of designing allocators would probably have been

struct MyAllocator { 
   template 
   class Core {
      // allocator code here
   };
};

Then it would have been possible to write

std::vector  vec;

rather than the somewhat misleading expression

std::vector  > vec;

I am not sure whether the above MyAllocator is permitted to be used as an allocator after adding a rebind, i.e. whether the following is a valid allocator class:

struct MyAllocator { 
   template 
   class Core {
      // allocator code here
   };

   template 
   struct rebind { using other=Core; };
};