20 General utilities library [utilities]

20.12 Memory resources [mem.res]

20.12.6 Class monotonic_­buffer_­resource [mem.res.monotonic.buffer]

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A monotonic_­buffer_­resource is a special-purpose memory resource intended for very fast memory allocations in situations where memory is used to build up a few objects and then is released all at once when the memory resource object is destroyed.
It has the following qualities:
  • (1.1)
    A call to deallocate has no effect, thus the amount of memory consumed increases monotonically until the resource is destroyed.
  • (1.2)
    The program can supply an initial buffer, which the allocator uses to satisfy memory requests.
  • (1.3)
    When the initial buffer (if any) is exhausted, it obtains additional buffers from an upstream memory resource supplied at construction. Each additional buffer is larger than the previous one, following a geometric progression.
  • (1.4)
    It is intended for access from one thread of control at a time. Specifically, calls to allocate and deallocate do not synchronize with one another.
  • (1.5)
    It frees the allocated memory on destruction, even if deallocate has not been called for some of the allocated blocks.
namespace std::pmr {
  class monotonic_buffer_resource : public memory_resource {
    memory_resource* upstream_rsrc;     // exposition only
    void* current_buffer;               // exposition only
    size_t next_buffer_size;            // exposition only

  public:
    explicit monotonic_buffer_resource(memory_resource* upstream);
    monotonic_buffer_resource(size_t initial_size, memory_resource* upstream);
    monotonic_buffer_resource(void* buffer, size_t buffer_size, memory_resource* upstream);

    monotonic_buffer_resource()
      : monotonic_buffer_resource(get_default_resource()) {}
    explicit monotonic_buffer_resource(size_t initial_size)
      : monotonic_buffer_resource(initial_size, get_default_resource()) {}
    monotonic_buffer_resource(void* buffer, size_t buffer_size)
      : monotonic_buffer_resource(buffer, buffer_size, get_default_resource()) {}

    monotonic_buffer_resource(const monotonic_buffer_resource&) = delete;

    virtual ~monotonic_buffer_resource();

    monotonic_buffer_resource& operator=(const monotonic_buffer_resource&) = delete;

    void release();
    memory_resource* upstream_resource() const;

  protected:
    void* do_allocate(size_t bytes, size_t alignment) override;
    void do_deallocate(void* p, size_t bytes, size_t alignment) override;

    bool do_is_equal(const memory_resource& other) const noexcept override;
  };
}

20.12.6.1 Constructors and destructor [mem.res.monotonic.buffer.ctor]

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explicit monotonic_buffer_resource(memory_resource* upstream); monotonic_buffer_resource(size_t initial_size, memory_resource* upstream);
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Preconditions: upstream is the address of a valid memory resource.
initial_­size, if specified, is greater than zero.
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Effects: Sets upstream_­rsrc to upstream and current_­buffer to nullptr.
If initial_­size is specified, sets next_­buffer_­size to at least initial_­size; otherwise sets next_­buffer_­size to an implementation-defined size.
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monotonic_buffer_resource(void* buffer, size_t buffer_size, memory_resource* upstream);
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Preconditions: upstream is the address of a valid memory resource.
buffer_­size is no larger than the number of bytes in buffer.
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Effects: Sets upstream_­rsrc to upstream, current_­buffer to buffer, and next_­buffer_­size to buffer_­size (but not less than 1), then increases next_­buffer_­size by an implementation-defined growth factor (which need not be integral).
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~monotonic_buffer_resource();
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Effects: Calls release().

20.12.6.2 Members [mem.res.monotonic.buffer.mem]

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void release();
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Effects: Calls upstream_­rsrc->deallocate() as necessary to release all allocated memory.
2
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[Note
:
The memory is released back to upstream_­rsrc even if some blocks that were allocated from this have not been deallocated from this.
— end note
]
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memory_resource* upstream_resource() const;
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Returns: The value of upstream_­rsrc.
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void* do_allocate(size_t bytes, size_t alignment) override;
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Returns: A pointer to allocated storage ([basic.stc.dynamic.allocation]) with a size of at least bytes.
The size and alignment of the allocated memory shall meet the requirements for a class derived from memory_­resource ([mem.res.class]).
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Effects: If the unused space in current_­buffer can fit a block with the specified bytes and alignment, then allocate the return block from current_­buffer; otherwise set current_­buffer to upstream_­rsrc->allocate(n, m), where n is not less than max(bytes, next_­buffer_­size) and m is not less than alignment, and increase next_­buffer_­size by an implementation-defined growth factor (which need not be integral), then allocate the return block from the newly-allocated current_­buffer.
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Throws: Nothing unless upstream_­rsrc->allocate() throws.
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void do_deallocate(void* p, size_t bytes, size_t alignment) override;
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Effects: None.
8
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Throws: Nothing.
9
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Remarks: Memory used by this resource increases monotonically until its destruction.
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bool do_is_equal(const memory_resource& other) const noexcept override;
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Returns: this == &other.