The Standard Librarian: What Are Allocators Good For?
By Matthew H. Austern, December 01, 2000
Most of us who use the C++ Standard library tend to forget about allocators, those mysterious things specified by default template parameters for STL containers. In most situations you will not need to call an allocator explicitly or write one of your own. But there are occasions when you might want to substitute your own custom allocator for the default version, for example, to allocate objects from a special memory pool. In this column Matt Austern discusses what you can use allocators for and how you can define your own.
December 2000 C++ Experts Forum/The Standard Librarian/Listing 1
template <class T> class malloc_allocator
{
public:
typedef T value_type;
typedef value_type* pointer;
typedef const value_type* const_pointer;
typedef value_type& reference;
typedef const value_type& const_reference;
typedef std::size_t size_type;
typedef std::ptrdiff_t difference_type;
template <class U>
struct rebind { typedef malloc_allocator<U> other; };
malloc_allocator() {}
malloc_allocator(const malloc_allocator&) {}
template <class U>
malloc_allocator(const malloc_allocator<U>&) {}
~malloc_allocator() {}
pointer address(reference x) const { return &x; }
const_pointer address(const_reference x) const {
return x;
}
pointer allocate(size_type n, const_pointer = 0) {
void* p = std::malloc(n * sizeof(T));
if (!p)
throw std::bad_alloc();
return static_cast<pointer>(p);
}
void deallocate(pointer p, size_type) { std::free(p); }
size_type max_size() const {
return static_cast<size_type>(-1) / sizeof(T);
}
void construct(pointer p, const value_type& x) {
new(p) value_type(x);
}
void destroy(pointer p) { p->~value_type(); }
private:
void operator=(const malloc_allocator&);
};
template<> class malloc_allocator<void>
{
typedef void value_type;
typedef void* pointer;
typedef const void* const_pointer;
template <class U>
struct rebind { typedef malloc_allocator<U> other; };
};
template <class T>
inline bool operator==(const malloc_allocator<T>&,
const malloc_allocator<T>&) {
return true;
}
template <class T>
inline bool operator!=(const malloc_allocator<T>&,
const malloc_allocator<T>&) {
return false;
}
