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One of the main complaints about the STL is the perceived lack of memory efficiency. In this article I’ll present a simple pooled allocator for use with the STL containers that allocate single elements at a time, such as list, set or map.

The Core of An STL Allocator

The core functions of an STL allocator are allocate and deallocate. These functions only deal with allocation, not with initialisation. Memory from these functions is then used in placement new operations within the container. This is to allow containers such as vector to allocate large chunks of memory, then do placement new in a loop for optimal efficiency. The declaration for these member functions is as follows:

// allocate a contiguous block of memory
pointer allocate(size_type count,
                 std::allocator::const_pointer hint = 0) const;
 
// deallocate a previously allocated block of memory
void deallocate(pointer block, size_type count) const throw();

The arguments are described as follows:

• count: the number of elements (not bytes) to allocate. The same number that was passed to a call to allocate is passed to the corresponding call to deallocate.
• hint: the location of a previously allocated (and not yet deallocated) block of memory from this allocator, or null. This pointer can be used to improve data locality if possible.
• block: an allocated block of memory due for destruction. This will never be null.
• allocate return value: a block of memory with at least enough space for count elements. If this memory cannot be allocated then throw the appropriate exception (such as std::bad_alloc).

Rebinding and Preserving State

An allocator must be able to bind itself to an alternate type. This is done by creating an allocator specifically for the desired type, then calling a constructor with the previous allocator as the argument.

// the type of an allocator for type U
template<typename U> 
struct rebind { typedef MyAllocator<U> other; };
 
// how to construct this allocator from an allocator for type U
template<typename U>
MyAllocator(MyAllocator<U> const& arg);

If your allocator has state (such as a pool location to allocate from), then the constructor in the code fragment above must do the necessary state management. Unfortunately there is no concept of a template friend class so any member variables must be made public, as will be shown in our pooled allocator implementation.

To test if memory allocated using one allocator can be freed by another, the standard also requires that we implement two operators:

// returns true if the allocators are interchangeable
template<typename T, typename U>
bool operator==(MyAllocator<T> const& left, MyAllocator<U> const& right);
 
// returns true if the allocators are not interchangeable
template<typename T, typename U>
bool operator!=(MyAllocator<T> const& left, MyAllocator<U> const& right);

In our pooled allocator implementation this simply compares the pool values: if they are equal (including both null), then memory from one allocator can be deallocated by the other.

Simple Pooling

Here is the interface to the pool class I’ll be using in the example:

// simple pool class that allows single allocations only
class Pool
{
public:
    Pool(size_t granularity, size_t size);
    ~Pool();
 
    // allocation/deallocation only (no construction)
    void* Allocate();
    void Deallocate(void* block);
 
    // construction/destruction included in the allocation
    template<typename T> T* Construct();
    template<typename T> void Destroy(T* instance);
 
    // pool status
    size_t GetGranularity() const;
    size_t GetSize() const;
    size_t GetUsed() const;
    size_t GetOverflow() const;
 
    // gets an allocator that uses this pool
    PooledAllocator<void> GetAllocator();
};

The functions are used as follows:

• Allocate/Deallocate: allocates/deallocates single elements from the pool. The return value from Allocate points to enough memory to store granularity bytes of data. Deallocate must not be passed a null pointer.
• Construct/Destruct: constructs/destructs objects from the pool. Behaves just like operator new/delete except will try to use pooled storage.
• GetAllocator: returns an STL-compliant pooled allocator that allocates from this pool.

Example Allocator Usage for std::list

Here’s a very short example C++ program that uses the pooled allocator:

#include <iostream>
#include "pooled_alloc.h"
#include <boost/scoped_ptr.hpp>
 
int main()
{
    // create a pool of 256 32-byte elements
    boost::scoped_ptr<Pool> pool(new Pool(32, 256));
 
    // use this pool with a std::list of floats
    PooledList<float>::Type test(pool->GetAllocator());
    test.push_back(42.0f);
}

Use The Source

Here’s the source to the pool class, the pooled allocator and some helpful structs that act as template typedefs (an annoying ommission from the standard). This code works with GCC 3.1 and Visual Studio 2003, so I’m pretty sure you could get it working with other compilers with minimal (if any) changes.

pool-source.zip

Obviously I won’t claim that the code is idiot-proof, but I’ve been using it my hobby projects so far without error. In the test cases I’ve tried, using a pooled allocator gains back up to 35% of the insertion/deletion cost compared with the default allocator.