Comparing allocators from two arenas

Understanding C++ memory allocators and arena allocation

This is a medium-difficulty C++ language-knowledge question that tests your grasp of custom memory allocators, arena-based allocation strategies, and how allocator equality comparisons work in the standard library. It requires you to trace through allocator construction, object placement, and the semantics of allocator comparison operators.

To work through this problem, you need to understand how allocators from different arenas relate to each other, what happens when you compare allocators for equality, and how that comparison affects container behaviour and memory management. The question rewards careful reading of allocator semantics and attention to detail around object lifetime and allocation source.

• Allocator interface and the operator== contract
• Arena-based memory pools and allocator instances
• Container equality and allocator propagation
• Lifetime and destruction guarantees