etl/docs/containers/pools/pool.md

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pool	3

{{< callout >}} Header: pool.h
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A fixed capacity object pool, where allocation and release are O(1) operations.

Internally defined storage

etl::pool<typename T, size_t Size>

Externally defined storage

etl::pool_ext<typename T>

---

etl::pool inherits from etl::generic_pool, which itself inherits from etl::ipool.
etl::ipool may be used as a size and type independent pointer or reference type for any etl::pool.

Notes
There are two methods for allocating objects from the pool.

allocate
release

allocate does not construct. It merely provides access to memory that is sized and aligned to contain a T object. The programmer must use placement new to construct the object.
release returns the memory allocation to the to the pool.

create
destroy

create allocates memory from the pool and calls its constructor.
destroy will call the destructor for the object and release it back to the pool.

Example

class Data
{
   ...
};

etl::pool<Data, 10> data_pool;

// Create.
Data* pdata = new (data_pool.allocate<Data>()) Data();

// Destroy
pdata->~Data();
data_pool.release(pdata);

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Heterogeneous pools may be constructed by basing the pool's type on a union, or using etl::variant.

union Data
{
  char   text[100];
  int    counter;
  double ratio;
};

etl::pool<Data, 10> data_pool;

char   *pc = data_pool.allocate<char>();
int    *pi = data_pool.allocate<int>();
double *pd = data_pool.allocate<double>();

Constructors

pool()

Description
For etl::pool.
Constructs a pool.
No elements are constructed.

---

pool_ext(char* buffer, size_t size)

Description
For etl::pool_ext. Constructs a pool from an external bufffer.
No elements are constructed.

Operations

template <typename T>
T* allocate()

Description
Allocates an item from the pool and returns a pointer to it.
If the pool has no free items then an etl::pool_no_allocation() is emitted and a nullptr is returned.
Note: Does not call the object's constructor.

---

void release(const void* const p_object);

Description
Releases an object back to the pool.
If the object does not belong to the pool an etl::pool_object_not_in_pool() is emitted.
Note: Does not call the object's destructor.

---

void release_all();

Releases all objects back to the pool.
Note: Does not destruct any T objects.

---

bool is_in_pool(const T* const p_object) const;

Description
Checks to see if an object belongs to the pool.
Returns true if it does, otherwise false.

---

C++03
Description

template <typename T>
T* create()

template <typename T, typename T1>
T* create(const T1& value1)

template <typename T, typename T1, typename T2>
T* create(const T1& value1, const T2& value2)

template <typename T, typename T1, typename T2, typename T3>
T* create(const T1& value1, const T2& value2, const T3& value3)

template <typename T, typename T1, typename T2, typename T3, typename T4>
T* create(const T1& value1, const T2& value2, const T3& value3, const T4& value4)

---

C++11

template <typename T, typename... Args>
T* create(Args&&... args)

---

There is a matching destroy function.

template <typename T>
void destroy(const void* const p_object)

Capacity

bool empty() const

Description
Returns true if there are no allocated objects in the pool, otherwise false.

---

bool full() const

Description
Returns true if there are no free objects in the pool, otherwise false.

---

size_t available() const

Description
Returns the remaining available free objects in the pool.

---

size_t size() const

Description
Returns the number of allocated objects in the pool.

---

size_t max_size() const

Description
Returns the maximum number of objects in the pool.

Constants

TYPE_SIZE The size of an item in the pool.
SIZE The maximum number of items in the pool.
ALIGNMENT The alignment of items in the pool.

The Technical Bit

The pool is based around a block of memory, with storage for Size items, properly aligned for type T.
Each item in the pool is a union of a uintptr_t and a type T. Free items contain a pointer to the next free item. Allocated items contain a T value.

Allocation is quick, as all that is necessary is to return the address of the next free item.

Release is similarly quick, as the item's content is simply replaced with the address of the current next free item.

On first use the memory block is uninitialised. On each new allocation a new item is initialised with the address of the next free item. This just-in-time initialisation means that construction does not involve writing to a potentially large amount of memory in one go.