23 General utilities library [utilities]

23.12 Memory resources [mem.res]

23.12.2 Class memory_­resource [mem.res.class]

The memory_­resource class is an abstract interface to an unbounded set of classes encapsulating memory resources.
namespace std::pmr {
  class memory_resource {
    static constexpr size_t max_align = alignof(max_align_t);   // exposition only

  public:
    virtual ~memory_resource();

    [[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
    void deallocate(void* p, size_t bytes, size_t alignment = max_align);

    bool is_equal(const memory_resource& other) const noexcept;

  private:
    virtual void* do_allocate(size_t bytes, size_t alignment) = 0;
    virtual void do_deallocate(void* p, size_t bytes, size_t alignment) = 0;

    virtual bool do_is_equal(const memory_resource& other) const noexcept = 0;
  };
}

23.12.2.1 memory_­resource public member functions [mem.res.public]

~memory_resource();
Effects: Destroys this memory_­resource.
[[nodiscard]] void* allocate(size_t bytes, size_t alignment = max_align);
Effects: Equivalent to: return do_­allocate(bytes, alignment);
void deallocate(void* p, size_t bytes, size_t alignment = max_align);
Effects: Equivalent to: do_­deallocate(p, bytes, alignment);
bool is_equal(const memory_resource& other) const noexcept;
Effects: Equivalent to: return do_­is_­equal(other);

23.12.2.2 memory_­resource private virtual member functions [mem.res.private]

virtual void* do_allocate(size_t bytes, size_t alignment) = 0;
Requires: alignment shall be a power of two.
Returns: A derived class shall implement this function to return a pointer to allocated storage ([basic.stc.dynamic.deallocation]) with a size of at least bytes.
The returned storage is aligned to the specified alignment, if such alignment is supported ([basic.align]); otherwise it is aligned to max_­align.
Throws: A derived class implementation shall throw an appropriate exception if it is unable to allocate memory with the requested size and alignment.
virtual void do_deallocate(void* p, size_t bytes, size_t alignment) = 0;
Requires: p shall have been returned from a prior call to allocate(bytes, alignment) on a memory resource equal to *this, and the storage at p shall not yet have been deallocated.
Effects: A derived class shall implement this function to dispose of allocated storage.
Throws: Nothing.
virtual bool do_is_equal(const memory_resource& other) const noexcept = 0;
Returns: A derived class shall implement this function to return true if memory allocated from this can be deallocated from other and vice-versa, otherwise false.
[Note
:
The most-derived type of other might not match the type of this.
For a derived class D, an implementation of this function could immediately return false if dynamic_­cast<const D*>(&other) == nullptr.
end note
]

23.12.2.3 memory_­resource equality [mem.res.eq]

bool operator==(const memory_resource& a, const memory_resource& b) noexcept;
Returns: &a == &b || a.is_­equal(b).
bool operator!=(const memory_resource& a, const memory_resource& b) noexcept;
Returns: !(a == b).