stl allocator, copy constructor of other type, rebind

Question

The STL allocators require this constructor form (20.1.5): X a(b); with the requirement that Y(a) == b;

In the standard implementation this implies, and is implemented as:

  template<class U> allocator( const allocator<U> & o ) throw()

I'm having trouble understanding why this requirement exists. I understand that allocators should be static (not have any state), but why on earth should you be able to convert them like this?

Answer 1

To allow construction from other allocators, as containers need to use a different allocator type than you specify. Lists and maps, for example, allocate their internal node type rather than the value_type they expose.

The code looks similar to:

template<class T, class Alloc=std::allocator<T> >
struct Container {
  typedef T value_type;
  typedef Alloc allocator_type;

private:
  struct Node {/*...*/};
  typedef typename Alloc::template rebind<Node>::other RealAllocatorType;
  RealAllocatorType allocator;
};