20 Memory management library [mem]

20.4 Memory resources [mem.res]

20.4.2 Class memory_resource [mem.res.class]

20.4.2.1 General [mem.res.class.general]

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The memory_resource class is an abstract interface to an unbounded set of classes encapsulating memory resources.
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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; }; }

20.4.2.2 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: Allocates storage by calling do_allocate(bytes, alignment) and implicitly creates objects within the allocated region of storage.
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Returns: A pointer to a suitable created object ([intro.object]) in the allocated region of storage.
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Throws: What and when the call to do_allocate throws.
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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);

20.4.2.3 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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Preconditions: alignment is 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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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.
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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 1: 
It is possible that the most-derived type of other does not match the type of this.
For a derived class D, an implementation of this function can immediately return false if dynamic_cast<const D*>(&other) == nullptr.
— end note]

20.4.2.4 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).