EDIT: after clarifications from the OP, it appears the original answer is off-topic; for reference's sake it is kept at the end of this answer.
Actually, the answer is rather simple: you simply need to keep a pointer both to the storage block and to the first item.
This does not, actually, requires a stateful allocator (it could be possible even in C++03, albeit with a custom std::align routine). The trick is that the allocator is not required to only ask of the system exactly enough memory to store user data. It can perfectly ask a bit more for book-keeping purposes of its own.
So, here we go creating an aligned allocator; to keep it simple I'll focus on the allocation/deallocation routines.
template <typename T>
class aligned_allocator {
// Allocates block of memory:
// - (opt) padding
// - offset: ptrdiff_t
// - T * n: T
// - (opt) padding
public:
typedef T* pointer;
typedef size_t size_type;
pointer allocate(size_type n);
void deallocate(pointer p, size_type n);
}; // class aligned_allocator
And now the allocation routine. Lots of memory fiddling, it's the heart of the allocator after all!
template <typename T>
auto aligned_allocator<T>::allocate(size_type n) -> pointer {
size_type const alignment = std::max(alignof(ptrdiff_t), alignof(T));
size_type const object_size = sizeof(ptrdiff_t) + sizeof(T)*n;
size_type const buffer_size = object_size + alignment;
// block is correctly aligned for `ptrdiff_t` because `std::malloc` returns
// memory correctly aligned for all built-ins types.
void* const block = std::malloc(buffer_size);
if (block == nullptr) { throw std::bad_alloc{}; }
// find the start of the body by suitably aligning memory,
// note that we reserve sufficient space for the header beforehand
void* storage = reinterpret_cast<char*>(block) + sizeof(ptrdiff_t);
size_t shift = buffer_size;
void* const body = std::align(alignment, object_size, storage, shift);
// reverse track to find where the offset field starts
char* const offset = reinterpret_cast<char*>(body) - sizeof(ptrdiff_t);
// store the value of the offset (ie, the result of body - block)
*reinterpret_cast<ptrdiff_t*>(offset) = sizeof(ptrdiff_t) + shift;
// finally return the start of the body
return reinterpret_cast<ptrdiff_t>(body);
} // aligned_allocator<T>::allocate
Fortunately the deallocation routine is much simpler, it just has to read the offset and apply it.
template <typename T>
void aligned_allocator<T>::deallocate(pointer p, size_type) {
// find the offset field
char const* header = reinterpret_cast<char*>(p) - sizeof(ptrdiff_t);
// read its value
ptrdiff_t const offset = *reinterpret_cast<ptrdiff_t*>(header);
// apply it to find start of block
void* const block = reinterpret_cast<char*>(p) - offset;
// finally deallocate
std::free(block);
} // aligned_allocator<T>::deallocate
The other routines need not be aware of the memory layout, so writing them is trivial.
Original answer:
template <typename T>
class Block {
public:
Block(Block const&) = delete;
Block& operator=(Block const&) = delete;
explicit Block(size_t n);
~Block();
private:
void* _storage;
T* _begin;
T* _end;
}; // class Block
template <typename T>
Block<T>::Block(size_t n) {
size_t const object_size = n * sizeof(T);
size_t const buffer_size = object_size + alignof(T);
_storage = std::malloc(size);
void* stock = _storage;
size_t shift = buffer_size;
std::align(alignof(T), object_size, stock, shift);
_begin = _end = reinterpret_cast<T*>(stock);
} // Block<T>::Block
template <typename T>
Block<T>::~Block() {
for (; _end != _begin; --_end) {
(_end - 1)->~T();
}
std::free(_storage);
} // Block<T>::~Block
