23 General utilities library [utilities]

23.12 Memory resources [mem.res]

23.12.5 Pool resource classes [mem.res.pool] Classes synchronized_­pool_­resource and unsynchronized_­pool_­resource [mem.res.pool.overview]

The synchronized_­pool_­resource and unsynchronized_­pool_­resource classes (collectively called pool resource classes) are general-purpose memory resources having the following qualities:
  • Each resource frees its allocated memory on destruction, even if deallocate has not been called for some of the allocated blocks.
  • A pool resource consists of a collection of pools, serving requests for different block sizes.
    Each individual pool manages a collection of chunks that are in turn divided into blocks of uniform size, returned via calls to do_­allocate.
    Each call to do_­allocate(size, alignment) is dispatched to the pool serving the smallest blocks accommodating at least size bytes.
  • When a particular pool is exhausted, allocating a block from that pool results in the allocation of an additional chunk of memory from the upstream allocator (supplied at construction), thus replenishing the pool.
    With each successive replenishment, the chunk size obtained increases geometrically.
    By allocating memory in chunks, the pooling strategy increases the chance that consecutive allocations will be close together in memory.
    end note
  • Allocation requests that exceed the largest block size of any pool are fulfilled directly from the upstream allocator.
  • A pool_­options struct may be passed to the pool resource constructors to tune the largest block size and the maximum chunk size.
A synchronized_­pool_­resource may be accessed from multiple threads without external synchronization and may have thread-specific pools to reduce synchronization costs.
An unsynchronized_­pool_­resource class may not be accessed from multiple threads simultaneously and thus avoids the cost of synchronization entirely in single-threaded applications.
namespace std::pmr {
  struct pool_options {
    size_t max_blocks_per_chunk = 0;
    size_t largest_required_pool_block = 0;

  class synchronized_pool_resource : public memory_resource {
    synchronized_pool_resource(const pool_options& opts, memory_resource* upstream);

        : synchronized_pool_resource(pool_options(), get_default_resource()) {}
    explicit synchronized_pool_resource(memory_resource* upstream)
        : synchronized_pool_resource(pool_options(), upstream) {}
    explicit synchronized_pool_resource(const pool_options& opts)
        : synchronized_pool_resource(opts, get_default_resource()) {}

    synchronized_pool_resource(const synchronized_pool_resource&) = delete;
    virtual ~synchronized_pool_resource();

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

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

    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;

  class unsynchronized_pool_resource : public memory_resource {
    unsynchronized_pool_resource(const pool_options& opts, memory_resource* upstream);

        : unsynchronized_pool_resource(pool_options(), get_default_resource()) {}
    explicit unsynchronized_pool_resource(memory_resource* upstream)
        : unsynchronized_pool_resource(pool_options(), upstream) {}
    explicit unsynchronized_pool_resource(const pool_options& opts)
        : unsynchronized_pool_resource(opts, get_default_resource()) {}

    unsynchronized_pool_resource(const unsynchronized_pool_resource&) = delete;
    virtual ~unsynchronized_pool_resource();

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

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

    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;