9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

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

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The attribute-token no_unique_address specifies that a non-static data member is a potentially-overlapping subobject ([intro.object]).
No attribute-argument-clause shall be present.
The attribute may appertain to a non-static data member other than a bit-field.
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[Note 1: 
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]
Recommended practice: The value of a has-attribute-expression for the no_unique_address attribute should be 0 for a given implementation unless this attribute can cause a potentially-overlapping subobject to have zero size.
[Example 1: 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]