20 General utilities library [utilities]

20.12 Memory resources [mem.res]

20.12.2 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:
    memory_resource() = default;
    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;
  };
}

20.12.2.1 Public member functions [mem.res.public]

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~memory_resource();

Effects: Destroys this memory_resource.

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

Effects: Equivalent to: return do_allocate(bytes, alignment);

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void deallocate(void* p, size_t bytes, size_t alignment = max_align);

Effects: Equivalent to do_deallocate(p, bytes, alignment).

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bool is_equal(const memory_resource& other) const noexcept;

Effects: Equivalent to: return do_is_equal(other);

20.12.2.2 Private virtual member functions [mem.res.private]

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virtual void* do_allocate(size_t bytes, size_t alignment) = 0;

Preconditions: alignment is a power of two.

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.

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;

Preconditions: p was returned from a prior call to allocate(bytes, alignment) on a memory resource equal to *this, and the storage at p has not yet been deallocated.

Effects: A derived class shall implement this function to dispose of allocated storage.

Throws: Nothing.

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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

]

20.12.2.3 Equality [mem.res.eq]

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bool operator==(const memory_resource& a, const memory_resource& b) noexcept;