9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.10 No unique address attribute [dcl.attr.nouniqueaddr]

1
#
The attribute-token no_­unique_­address specifies that a non-static data member is a potentially-overlapping subobject ([intro.object]).
It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
The attribute may appertain to a non-static data member other than a bit-field.
2
#
[Note
:
The non-static data member can share the address of another non-static data member or that of a base class, and any padding that would normally be inserted at the end of the object can be reused as storage for other members.
— end note
]
[Example
: 
template<typename Key, typename Value,
         typename Hash, typename Pred, typename Allocator>
class hash_map {
  [[no_unique_address]] Hash hasher;
  [[no_unique_address]] Pred pred;
  [[no_unique_address]] Allocator alloc;
  Bucket *buckets;
  // ...
public:
  // ...
};
Here, hasher, pred, and alloc could have the same address as buckets if their respective types are all empty.
— end example
]