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.

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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 Tp* allocate(size_t n); void deallocate(Tp* p, size_t n); 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> T* allocate_object(size_t n = 1); template<class T> void deallocate_object(T* p, size_t n = 1); template<class T, class... CtorArgs> 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); template<class T> void destroy(T* p); 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(); } }; }

20.4.3.2 Constructors [mem.poly.allocator.ctor]

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polymorphic_allocator() noexcept;

Effects: Sets memory_rsrc to get_default_resource().

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polymorphic_allocator(memory_resource* r);

Preconditions: r is non-null.

Effects: Sets memory_rsrc to r.

Throws: Nothing.

[Note 1:

This constructor provides an implicit conversion from memory_resource*.

— end note]

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template<class U> polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept;

Effects: Sets memory_rsrc to other.resource().

20.4.3.3 Member functions [mem.poly.allocator.mem]

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Tp* allocate(size_t n);

Effects: If numeric_limits<size_t>::max() / sizeof(Tp) < n, throws bad_array_new_length.

Otherwise equivalent to: return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));

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void deallocate(Tp* p, size_t n);

Preconditions: p was allocated from a memory resource x, equal to *memory_rsrc, using x.allocate(n * sizeof(Tp), alignof(Tp)).

Effects: Equivalent to memory_rsrc->deallocate(p, n * sizeof(Tp), alignof(Tp)).

Throws: Nothing.

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void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));

Effects: Equivalent to: return memory_rsrc->allocate(nbytes, alignment);

[Note 1:

The return type is void* (rather than, e.g., byte*) to support conversion to an arbitrary pointer type U* by static_cast<U*>, thus facilitating construction of a U object in the allocated memory.

— end note]

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void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t));

Effects: Equivalent to memory_rsrc->deallocate(p, nbytes, alignment).

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template<class T>
  T* allocate_object(size_t n = 1);

Effects: Allocates memory suitable for holding an array of n objects of type T, as follows:

(8.1)
if numeric_limits<size_t>::max() / sizeof(T) < n, throws bad_array_new_length,
(8.2)
otherwise equivalent to: return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));

[Note 2:

T is not deduced and must therefore be provided as a template argument.

— end note]

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template<class T>
  void deallocate_object(T* p, size_t n = 1);

Effects: Equivalent to deallocate_bytes(p, n*sizeof(T), alignof(T)).

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template<class T, class... CtorArgs>
  T* new_object(CtorArgs&&... ctor_args);

Effects: Allocates and constructs an object of type T, as follows.

Equivalent to: T* p = allocate_object<T>(); try { construct(p, std::forward<CtorArgs>(ctor_args)...); } catch (...) { deallocate_object(p); throw; } return p;

[Note 3:

T is not deduced and must therefore be provided as a template argument.

— end note]

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template<class T>
  void delete_object(T* p);

Effects: Equivalent to: destroy(p); deallocate_object(p);

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template<class T, class... Args>
  void construct(T* p, Args&&... args);

Mandates: Uses-allocator construction of T with allocator *this (see [allocator.uses.construction]) and constructor arguments std::forward<Args>(args)... is well-formed.

Effects: Construct a T object in the storage whose address is represented by p by uses-allocator construction with allocator *this and constructor arguments std::forward<Args>(args)....

Throws: Nothing unless the constructor for T throws.

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template<class T>
  void destroy(T* p);

Effects: Equivalent to p->~T().

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polymorphic_allocator select_on_container_copy_construction() const;

Returns: polymorphic_allocator().

[Note 4:

The memory resource is not propagated.

— end note]

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memory_resource* resource() const;

Returns: memory_rsrc.

20.4.3.4 Equality [mem.poly.allocator.eq]

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template<class T1, class T2>
  bool operator==(const polymorphic_allocator<T1>& a,
                  const polymorphic_allocator<T2>& b) noexcept;

Returns: *a.resource() == *b.resource().