20 Memory management library [mem]

20.4 Memory resources [mem.res]

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

20.4.6.1 General [mem.res.monotonic.buffer.general]

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.

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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; }; }

20.4.6.2 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);

Preconditions: upstream is the address of a valid memory resource.

initial_size, if specified, is greater than zero.

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);

Preconditions: upstream is the address of a valid memory resource.

buffer_size is no larger than the number of bytes in buffer.

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();

Effects: Calls release().

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

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void release();

Effects: Calls upstream_rsrc->deallocate() as necessary to release all allocated memory.

Resets current_buffer and next_buffer_size to their initial values at construction.

[Note 1:

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;

Returns: The value of upstream_rsrc.

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void* do_allocate(size_t bytes, size_t alignment) override;

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.

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]).

Throws: Nothing unless upstream_rsrc->allocate() throws.

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

Effects: None.

Throws: Nothing.

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;

Returns: this == &other.