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## Class template singleton_pool

boost::singleton_pool

## Synopsis

    // In header: <boost/pool/singleton_pool.hpp>

    template<typename Tag, unsigned RequestedSize, typename UserAllocator,
             typename Mutex, unsigned NextSize, unsigned MaxSize>
    class singleton_pool {
    public:
      // types
      typedef Tag                                    tag;
      typedef Mutex                                  mutex;            // The type of mutex used to synchonise access to this pool (default details::pool::default_mutex).
      typedef UserAllocator                          user_allocator;   // The user-allocator used by this pool, default = default_user_allocator_new_delete.
      typedef pool< UserAllocator >::size_type       size_type;        // size_type of user allocator.
      typedef pool< UserAllocator >::difference_type difference_type;  // difference_type of user allocator.

      // member classes/structs/unions

      struct object_creator {
        // construct/copy/destruct
        object_creator();

        // public member functions
        void do_nothing() const;
      };

      // construct/copy/destruct
      singleton_pool();

      // public static functions
      static void * malloc();
      static void * ordered_malloc();
      static void * ordered_malloc(const size_type);
      static bool is_from(void *const);
      static void free(void *const);
      static void ordered_free(void *const);
      static void free(void *const, const size_type);
      static void ordered_free(void *const, const size_type);
      static bool release_memory();
      static bool purge_memory();

      // private static functions
      static pool_type & get_pool();

      // public data members
      static const unsigned requested_size;  // The size of each chunk allocated by this pool.
      static const unsigned next_size;  // The number of chunks to allocate on the first allocation.
      static pool< UserAllocator > p;  // For exposition only!
    };

## Description

The singleton_pool class allows other pool interfaces for types of the same size to share the same pool. Template parameters are as follows:

Tag User-specified type to uniquely identify this pool: allows different unbounded sets of singleton pools to exist.

RequestedSize The size of each chunk returned by member function `malloc()`.

UserAllocator User allocator, default = default_user_allocator_new_delete.

Mutex This class is the type of mutex to use to protect simultaneous access to the underlying Pool. Can be any Boost.Thread Mutex type or `boost::details::pool::null_mutex`. It is exposed so that users may declare some singleton pools normally (i.e., with synchronization), but some singleton pools without synchronization (by specifying `boost::details::pool::null_mutex`) for efficiency reasons. The member typedef `mutex` exposes the value of this template parameter. The default for this parameter is boost::details::pool::default_mutex which is a synonym for either `boost::details::pool::null_mutex` (when threading support is turned off in the compiler (so BOOST_HAS_THREADS is not set), or threading support has ben explicitly disabled with BOOST_DISABLE_THREADS (Boost-wide disabling of threads) or BOOST_POOL_NO_MT (this library only)) or for `boost::mutex` (when threading support is enabled in the compiler).

NextSize The value of this parameter is passed to the underlying Pool when it is created and specifies the number of chunks to allocate in the first allocation request (defaults to 32). The member typedef `static const value next_size` exposes the value of this template parameter.

MaxSizeThe value of this parameter is passed to the underlying Pool when it is created and specifies the maximum number of chunks to allocate in any single allocation request (defaults to 0).

Notes:

The underlying pool p referenced by the static functions in singleton_pool is actually declared in a way that is:

1 Thread-safe if there is only one thread running before main() begins and after main() ends – all of the static functions of singleton_pool synchronize their access to p.

2 Guaranteed to be constructed before it is used – thus, the simple static object in the synopsis above would actually be an incorrect implementation. The actual implementation to guarantee this is considerably more complicated.

3 Note too that a different underlying pool p exists for each different set of template parameters, including implementation-specific ones.

4 The underlying pool is constructed "as if" by:

pool<UserAllocator> p(RequestedSize, NextSize, MaxSize);

| Note
The underlying pool constructed by the singleton is never freed. This means that memory allocated by a singleton_pool can be still used after main() has completed, but may mean that some memory checking programs will complain about leaks from singleton_pool.

### `singleton_pool` public types

1. typedef Tag tag;

   The Tag template parameter uniquely identifies this pool and allows different unbounded sets of singleton pools to exist. For example, the pool allocators use two tag classes to ensure that the two different allocator types never share the same underlying singleton pool. Tag is never actually used by singleton_pool.

### `singleton_pool` public construct/copy/destruct

1.     singleton_pool();

### `singleton_pool` public static functions

1.     static void * malloc();

   Equivalent to SingletonPool::p.malloc(); synchronized.

2.     static void * ordered_malloc();

   Equivalent to SingletonPool::p.ordered_malloc(); synchronized.

3.     static void * ordered_malloc(const size_type n);

   Equivalent to SingletonPool::p.ordered_malloc(n); synchronized.

4.     static bool is_from(void *const ptr);

   Equivalent to SingletonPool::p.is_from(chunk); synchronized.

   Returns: | true if chunk is from SingletonPool::is_from(chunk)

5.     static void free(void *const ptr);

   Equivalent to SingletonPool::p.free(chunk); synchronized.

6.     static void ordered_free(void *const ptr);

   Equivalent to SingletonPool::p.ordered_free(chunk); synchronized.

7.     static void free(void *const ptr, const size_type n);

   Equivalent to SingletonPool::p.free(chunk, n); synchronized.

8.     static void ordered_free(void *const ptr, const size_type n);

   Equivalent to SingletonPool::p.ordered_free(chunk, n); synchronized.

9.     static bool release_memory();

   Equivalent to SingletonPool::p.release_memory(); synchronized.

10.     static bool purge_memory();

    Equivalent to SingletonPool::p.purge_memory(); synchronized.

### `singleton_pool` private static functions

1.     static pool_type & get_pool();