A C++ template library for embedded applications
Designed and maintained by
Aster Consulting Ltd

pool

A fixed capacity object pool, where allocation and release are O(1) operations.
STL equivalent: none

etl::pool<typename T, const size_t SIZE>
etl::generic_pool<const size_t TYPE_SIZE, const size_t ALIGNMENT, const size_t SIZE>

etl::generic_pool Inherits from etl::ipool
etl::pool Inherits from etl::generic_pool
etl::ipool may be used as a size and type independent pointer or reference type for any etl::pool or
etl::generic_pool instance.

Notes

When using etl::generic_pool, TYPE_SIZE must be set to the size of the largest type to be stored, and ALIGNMENT set to
the largest alignment of all of the types to be stored.
This may be achieved using etl::largest.

typedef etl::largest<uint8_t, uint32_t, double> largest;
etl::generic_pool<largest::size, largest::alignment, 4> pool;

When using allocate the pool does not construct or destruct anything. It merely provides access to memory that is
sized and aligned to contain a T object, or any type that has a compatible alignment, in the case of etl::generic_pool.
It is up to the user of the pool to handle the lifetime of the object. Objects are  returned to the pool by calling release.

Objects can be constructed by calling a create function which will call the objects constructor. Calling destroy will call
the destructor for the object and release it back to the pool.

class Data
{
   ...
};

etl::pool<Data, 10> data_pool;

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

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

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

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>();

Constructor

pool()
generic_pool()
Constructs a pool. No elements are constructed.

Operations

template <typename T>
T* allocate()
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* pobject);
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* p_object) const;
Checks to see if an object belongs to the pool.
Returns true if it does, otherwise false.

If C++11 or later is supported then a variadic function is defined for creation.
template <typename T, typename... Args>
T* create(Args&&... args)

If C++ is not supported then create functions for up to four parameters are defined.
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)

There is a matching destroy function.
template <typename T>
void destroy(const void* const p_object)

Capacity

bool empty() const
Returns true if there are no allocated objects in the pool, otherwise false.

bool full() const
Returns true if there are no free objects in the pool, otherwise false.

size_t available() const
Returns the remaining available free objects in the pool.

size_t size() const
Returns the number of allocated objects in the pool.

size_t max_size() const
Returns the maximum number of objects in the pool.

Constants

etl::pool

SIZE      The maximum number of items in the pool.
ALIGNMENT The alignment of items in the pool.

etl::generic_pool

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

Technical stuff

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
also very 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.
pool.h