20 Memory management library [mem]

20.4 Memory resources [mem.res]

20.4.3 Class template polymorphic_­allocator [mem.poly.allocator.class]

20.4.3.1 General [mem.poly.allocator.class.general]

A specialization of class template pmr​::​polymorphic_­allocator meets the Cpp17Allocator requirements ([allocator.requirements.general]) if its template argument is a cv-unqualified object type.
Constructed with different memory resources, different instances of the same specialization of pmr​::​polymorphic_­allocator can exhibit entirely different allocation behavior.
This runtime polymorphism allows objects that use polymorphic_­allocator to behave as if they used different allocator types at run time even though they use the same static allocator type.
A specialization of class template pmr​::​polymorphic_­allocator meets the allocator completeness requirements ([allocator.requirements.completeness]) if its template argument is a cv-unqualified object type.
namespace std::pmr { template<class Tp = byte> class polymorphic_allocator { memory_resource* memory_rsrc; // exposition only public: using value_type = Tp; // [mem.poly.allocator.ctor], constructors polymorphic_allocator() noexcept; polymorphic_allocator(memory_resource* r); polymorphic_allocator(const polymorphic_allocator& other) = default; template<class U> polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept; polymorphic_allocator& operator=(const polymorphic_allocator&) = delete; // [mem.poly.allocator.mem], member functions [[nodiscard]] Tp* allocate(size_t n); void deallocate(Tp* p, size_t n); [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t)); void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t)); template<class T> [[nodiscard]] T* allocate_object(size_t n = 1); template<class T> void deallocate_object(T* p, size_t n = 1); template<class T, class... CtorArgs> [[nodiscard]] T* new_object(CtorArgs&&... ctor_args); template<class T> void delete_object(T* p); template<class T, class... Args> void construct(T* p, Args&&... args); polymorphic_allocator select_on_container_copy_construction() const; memory_resource* resource() const; // friends friend bool operator==(const polymorphic_allocator& a, const polymorphic_allocator& b) noexcept { return *a.resource() == *b.resource(); } }; }