19 General utilities library [utilities]

19.12 Memory resources [mem.res]

19.12.2 Class memory_­resource [mem.res.class]

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#
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:
    memory_resource(const memory_resource&) = default;
    virtual ~memory_resource();

    memory_resource& operator=(const memory_resource&) = default;

    [[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;
  };
}

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

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

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

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virtual void* do_allocate(size_t bytes, size_t alignment) = 0;
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Requires: alignment shall be a power of two.
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Returns: A derived class shall implement this function to return a pointer to allocated storage ([basic.stc.dynamic.allocation]) with a size of at least bytes, aligned to the specified alignment.
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Throws: A derived class implementation shall throw an appropriate exception if it is unable to allocate memory with the requested size and alignment.
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virtual void do_deallocate(void* p, size_t bytes, size_t alignment) = 0;
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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.
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Effects: A derived class shall implement this function to dispose of allocated storage.
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Throws: Nothing.
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virtual bool do_is_equal(const memory_resource& other) const noexcept = 0;
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#
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
 ]

19.12.2.3 memory_­resource equality [mem.res.eq]

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