User Allocators

Introduction

Pool objects need to request memory blocks from the system, which the Pool then splits into chunks to allocate to the user. By specifying a UserAllocator template parameter to various Pool interfaces, users can control how those system memory blocks are allocated.

Semantics

*Symbol Table*
Symbol	Meaning
UserAllocator	A User Allocator type
block	value of type char *
n	value of type UserAllocator::size_type

*Typedefs*
Expression	Type
UserAllocator::size_type	An unsigned integral type that can represent the size of the largest object to be allocated
UserAllocator::difference_type	A signed integral type that can represent the difference of any two pointers

*Allocation and Deallocation*
Expression	Return Type	Pre-Condition/Notes
UserAllocator::malloc(n)	char *	Attempts to allocate n bytes from the system. Returns 0 if out-of-memory.
UserAllocator::free(block)	void	block must have been previously returned from a call to UserAllocator::malloc.

Provided Implementations

There are two UserAllocator classes provided. Both of them are in pool.hpp (see pool). The default value for the template parameter UserAllocator is always default_user_allocator_new_delete.

Synopsis

struct default_user_allocator_new_delete
{
  typedef std::size_t size_type;
  typedef std::ptrdiff_t difference_type;

  static char * malloc(const size_type bytes)
  { return new (std::nothrow) char[bytes]; }
  static void free(char * const block)
  { delete [] block; }
};

struct default_user_allocator_malloc_free
{
  typedef std::size_t size_type;
  typedef std::ptrdiff_t difference_type;

  static char * malloc(const size_type bytes)
  { return reinterpret_cast<char *>(std::malloc(bytes)); }
  static void free(char * const block)
  { std::free(block); }
};

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