20 Memory management library [mem]

20.4 Memory resources [mem.res]

20.4.1 Header <memory_resource> synopsis [mem.res.syn]

namespace std::pmr { // [mem.res.class], class memory_resource class memory_resource; bool operator==(const memory_resource& a, const memory_resource& b) noexcept; // [mem.poly.allocator.class], class template polymorphic_allocator template<class Tp = byte> class polymorphic_allocator; template<class T1, class T2> bool operator==(const polymorphic_allocator<T1>& a, const polymorphic_allocator<T2>& b) noexcept; // [mem.res.global], global memory resources memory_resource* new_delete_resource() noexcept; memory_resource* null_memory_resource() noexcept; memory_resource* set_default_resource(memory_resource* r) noexcept; memory_resource* get_default_resource() noexcept; // [mem.res.pool], pool resource classes struct pool_options; class synchronized_pool_resource; class unsynchronized_pool_resource; class monotonic_buffer_resource; }

20.4.2 Class memory_resource [mem.res.class]

20.4.2.1 General [mem.res.class.general]

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();

Effects: Destroys this memory_resource.

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

Effects: Allocates storage by calling do_allocate(bytes, alignment) and implicitly creates objects within the allocated region of storage.

Returns: A pointer to a suitable created object ([intro.object]) in the allocated region of storage.

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);

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);

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

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

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 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;

Returns: &a == &b || a.is_equal(b).

20.4.3 Class template polymorphic_allocator [mem.poly.allocator.class]

20.4.3.1 General [mem.poly.allocator.class.general]

A specialization of class template pmr::polymorphic_allocator meets the Cpp17Allocator requirements ([allocator.requirements.general]) if its template argument is a cv-unqualified object type.

Constructed with different memory resources, different instances of the same specialization of pmr::polymorphic_allocator can exhibit entirely different allocation behavior.

This runtime polymorphism allows objects that use polymorphic_allocator to behave as if they used different allocator types at run time even though they use the same static allocator type.

A specialization of class template pmr::polymorphic_allocator meets the allocator completeness requirements ([allocator.requirements.completeness]) if its template argument is a cv-unqualified object type.

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namespace std::pmr { template<class Tp = byte> class polymorphic_allocator { memory_resource* memory_rsrc; // exposition only public: using value_type = Tp; // [mem.poly.allocator.ctor], constructors polymorphic_allocator() noexcept; polymorphic_allocator(memory_resource* r); polymorphic_allocator(const polymorphic_allocator& other) = default; template<class U> polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept; polymorphic_allocator& operator=(const polymorphic_allocator&) = delete; // [mem.poly.allocator.mem], member functions [[nodiscard]] Tp* allocate(size_t n); void deallocate(Tp* p, size_t n); [[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t)); void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t)); template<class T> [[nodiscard]] T* allocate_object(size_t n = 1); template<class T> void deallocate_object(T* p, size_t n = 1); template<class T, class... CtorArgs> [[nodiscard]] T* new_object(CtorArgs&&... ctor_args); template<class T> void delete_object(T* p); template<class T, class... Args> void construct(T* p, Args&&... args); polymorphic_allocator select_on_container_copy_construction() const; memory_resource* resource() const; // friends friend bool operator==(const polymorphic_allocator& a, const polymorphic_allocator& b) noexcept { return *a.resource() == *b.resource(); } }; }

20.4.3.2 Constructors [mem.poly.allocator.ctor]

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polymorphic_allocator() noexcept;

Effects: Sets memory_rsrc to get_default_resource().

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polymorphic_allocator(memory_resource* r);

Preconditions: r is non-null.

Effects: Sets memory_rsrc to r.

Throws: Nothing.

[Note 1:

This constructor provides an implicit conversion from memory_resource*.

— end note]

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template<class U> polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept;

Effects: Sets memory_rsrc to other.resource().

20.4.3.3 Member functions [mem.poly.allocator.mem]

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[[nodiscard]] Tp* allocate(size_t n);

Effects: If numeric_limits<size_t>::max() / sizeof(Tp) < n, throws bad_array_new_length.

Otherwise equivalent to: return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));

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void deallocate(Tp* p, size_t n);

Preconditions: p was allocated from a memory resource x, equal to *memory_rsrc, using x.allocate(n * sizeof(Tp), alignof(Tp)).

Effects: Equivalent to memory_rsrc->deallocate(p, n * sizeof(Tp), alignof(Tp)).

Throws: Nothing.

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[[nodiscard]] void* allocate_bytes(size_t nbytes, size_t alignment = alignof(max_align_t));

Effects: Equivalent to: return memory_rsrc->allocate(nbytes, alignment);

[Note 1:

The return type is void* (rather than, e.g., byte*) to support conversion to an arbitrary pointer type U* by static_cast<U*>, thus facilitating construction of a U object in the allocated memory.

— end note]

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void deallocate_bytes(void* p, size_t nbytes, size_t alignment = alignof(max_align_t));

Effects: Equivalent to memory_rsrc->deallocate(p, nbytes, alignment).

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template<class T>
  [[nodiscard]] T* allocate_object(size_t n = 1);

Effects: Allocates memory suitable for holding an array of n objects of type T, as follows:

(8.1)
if numeric_limits<size_t>::max() / sizeof(T) < n, throws bad_array_new_length,
(8.2)
otherwise equivalent to: return static_cast<T*>(allocate_bytes(n*sizeof(T), alignof(T)));

[Note 2:

T is not deduced and must therefore be provided as a template argument.

— end note]

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template<class T>
  void deallocate_object(T* p, size_t n = 1);

Effects: Equivalent to deallocate_bytes(p, n*sizeof(T), alignof(T)).

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template<class T, class... CtorArgs>
  [[nodiscard]] T* new_object(CtorArgs&&... ctor_args);

Effects: Allocates and constructs an object of type T, as follows.

Equivalent to: T* p = allocate_object<T>(); try { construct(p, std::forward<CtorArgs>(ctor_args)...); } catch (...) { deallocate_object(p); throw; } return p;

[Note 3:

T is not deduced and must therefore be provided as a template argument.

— end note]

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template<class T>
  void delete_object(T* p);

Effects: Equivalent to: allocator_traits<polymorphic_allocator>::destroy(*this, p); deallocate_object(p);

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template<class T, class... Args>
  void construct(T* p, Args&&... args);

Mandates: Uses-allocator construction of T with allocator *this (see [allocator.uses.construction]) and constructor arguments std::forward<Args>(args)... is well-formed.

Effects: Construct a T object in the storage whose address is represented by p by uses-allocator construction with allocator *this and constructor arguments std::forward<Args>(args)....

Throws: Nothing unless the constructor for T throws.

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polymorphic_allocator select_on_container_copy_construction() const;

Returns: polymorphic_allocator().

[Note 4:

The memory resource is not propagated.

— end note]

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memory_resource* resource() const;

Returns: memory_rsrc.

20.4.3.4 Equality [mem.poly.allocator.eq]

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template<class T1, class T2>
  bool operator==(const polymorphic_allocator<T1>& a,
                  const polymorphic_allocator<T2>& b) noexcept;

Returns: *a.resource() == *b.resource().

20.4.4 Access to program-wide memory_resource objects [mem.res.global]

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memory_resource* new_delete_resource() noexcept;

Returns: A pointer to a static-duration object of a type derived from memory_resource that can serve as a resource for allocating memory using ::operator new and ::operator delete.

The same value is returned every time this function is called.

For a return value p and a memory resource r, p->is_equal(r) returns &r == p.

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memory_resource* null_memory_resource() noexcept;

Returns: A pointer to a static-duration object of a type derived from memory_resource for which allocate() always throws bad_alloc and for which deallocate() has no effect.

The same value is returned every time this function is called.

For a return value p and a memory resource r, p->is_equal(r) returns &r == p.

The default memory resource pointer is a pointer to a memory resource that is used by certain facilities when an explicit memory resource is not supplied through the interface.

Its initial value is the return value of new_delete_resource().

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memory_resource* set_default_resource(memory_resource* r) noexcept;

Effects: If r is non-null, sets the value of the default memory resource pointer to r, otherwise sets the default memory resource pointer to new_delete_resource().

Returns: The previous value of the default memory resource pointer.

Remarks: Calling the set_default_resource and get_default_resource functions shall not incur a data race.

A call to the set_default_resource function shall synchronize with subsequent calls to the set_default_resource and get_default_resource functions.

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memory_resource* get_default_resource() noexcept;

Returns: The current value of the default memory resource pointer.

20.4.5 Pool resource classes [mem.res.pool]

20.4.5.1 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:

(1.1)
Each resource frees its allocated memory on destruction, even if deallocate has not been called for some of the allocated blocks.
(1.2)
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.
(1.3)
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.

[Note 1:
By allocating memory in chunks, the pooling strategy increases the chance that consecutive allocations will be close together in memory.
— end note]
(1.4)
Allocation requests that exceed the largest block size of any pool are fulfilled directly from the upstream allocator.
(1.5)
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.

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

20.4.5.2 pool_options data members [mem.res.pool.options]

The members of pool_options comprise a set of constructor options for pool resources.

The effect of each option on the pool resource behavior is described below:

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size_t max_blocks_per_chunk;

The maximum number of blocks that will be allocated at once from the upstream memory resource ([mem.res.monotonic.buffer]) to replenish a pool.

If the value of max_blocks_per_chunk is zero or is greater than an implementation-defined limit, that limit is used instead.

The implementation may choose to use a smaller value than is specified in this field and may use different values for different pools.

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size_t largest_required_pool_block;

The largest allocation size that is required to be fulfilled using the pooling mechanism.

Attempts to allocate a single block larger than this threshold will be allocated directly from the upstream memory resource.

If largest_required_pool_block is zero or is greater than an implementation-defined limit, that limit is used instead.

The implementation may choose a pass-through threshold larger than specified in this field.

20.4.5.3 Constructors and destructors [mem.res.pool.ctor]

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synchronized_pool_resource(const pool_options& opts, memory_resource* upstream);
unsynchronized_pool_resource(const pool_options& opts, memory_resource* upstream);

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

Effects: Constructs a pool resource object that will obtain memory from upstream whenever the pool resource is unable to satisfy a memory request from its own internal data structures.

The resulting object will hold a copy of upstream, but will not own the resource to which upstream points.

[Note 1:

The intention is that calls to upstream->allocate() will be substantially fewer than calls to this->allocate() in most cases.

— end note]

The behavior of the pooling mechanism is tuned according to the value of the opts argument.

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

It is unspecified if, or under what conditions, this constructor calls upstream->allocate().

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virtual ~synchronized_pool_resource();
virtual ~unsynchronized_pool_resource();

Effects: Calls release().

20.4.5.4 Members [mem.res.pool.mem]

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void release();

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

[Note 1:

The memory is released back to upstream_resource() even if deallocate has not been called for some of the allocated blocks.

— end note]

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memory_resource* upstream_resource() const;

Returns: The value of the upstream argument provided to the constructor of this object.

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pool_options options() const;

Returns: The options that control the pooling behavior of this resource.

The values in the returned struct may differ from those supplied to the pool resource constructor in that values of zero will be replaced with implementation-defined defaults, and sizes may be rounded to unspecified granularity.

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

Effects: If the pool selected for a block of size bytes is unable to satisfy the memory request from its own internal data structures, it will call upstream_resource()->allocate() to obtain more memory.

If bytes is larger than that which the largest pool can handle, then memory will be allocated using upstream_resource()->allocate().

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_resource()->allocate() throws.

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

Effects: Returns the memory at p to the pool.

It is unspecified if, or under what circumstances, this operation will result in a call to upstream_resource()->deallocate().

Throws: Nothing.

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

Returns: this == &other.

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.