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heap_allocator - heap-based allocator

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heap_allocator - heap-based allocator


Heap allocators are generally used when there is a lot of allocation and deallocation of small objects. For instance, this is often the case when dealing with std::list and std::map.
Heap-based allocator is conform to the STL specification of allocators. It does not "free" the memory until the heap is destroyed.
This allocator handles an a priori unlimited area of memory: a sequence of growing chunks are allocated. For a limited memory handler in the same spirit, see "stack_allocator"(9).


    typedef map <size_t, double, less<size_t>, heap_allocator<pair<size_t,double> > >  map_type;
    map_type a;
    a.insert (make_pair (0, 3.14));
    a.insert (make_pair (1, 1.17));
    for (map_type::iterator iter = a.begin(), last = a.end(); iter != last; iter++) {
      cout << (*iter).first << " " << (*iter).second << endl;


template <typename T>
class heap_allocator {
    struct handler_type; // forward declaration:
// typedefs:
typedef size_t size_type; typedef std::ptrdiff_t difference_type; typedef T* pointer; typedef const T* const_pointer; typedef T& reference; typedef const T& const_reference; typedef T value_type;
// constructors:
heap_allocator() throw() : handler (new handler_type) { } heap_allocator (const heap_allocator& ha) throw() : handler (ha.handler) { ++handler->reference_count; } template <typename U> heap_allocator (const heap_allocator<U>& ha) throw() : handler ((typename heap_allocator<T>::handler_type*)(ha.handler)) { ++handler->reference_count; } ~heap_allocator() throw() { check_macro (handler != NULL, "unexpected null mem_info"); if (--handler->reference_count == 0) delete handler; } // Rebind to allocators of other types template <typename U> struct rebind { typedef heap_allocator<U> other; };
// assignement:
heap_allocator& operator= (const heap_allocator& ha) { handler = ha.handler; ++handler->reference_count; return *this; }
// utility functions:
pointer address (reference r) const { return &r; } const_pointer address (const_reference c) const { return &c; } size_type max_size() const { return std::numeric_limits<size_t>::max() / sizeof(T); }
// in-place construction/destruction
void construct (pointer p, const_reference c) { // placement new operator: new( reinterpret_cast<void*>(p) ) T(c); } // C++ 2011: default construct a value of type T at the location referenced by p void construct (pointer p) { new ( reinterpret_cast<void*>(p) ) T(); }
void destroy (pointer p) { // call destructor directly: (p)->~T(); }
// allocate raw memory
pointer allocate (size_type n, const void* = NULL) { return pointer (handler->raw_allocate (n*sizeof(T))); } void deallocate (pointer p, size_type n) { // No need to free heap memory } const handler_type* get_handler() const { return handler; }
// data:
protected: handler_type* handler; template <typename U> friend class heap_allocator; };
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