intull intull - 2 months ago 13x
C++ Question

std::align and std::aligned_storage for aligned allocation of memory blocks

I'm trying to allocate a block of memory of size

which needs to be
aligned where the size may not be defined at compile time. I know routines such as
, etc exist but I do not want to use them as they bring down code portability.

C++11 gives a routine
and also a class
from which I can retrieve a POD type to allocate an element which will be aligned to my requirements. However my goal is to create an allocator which would allocate a block of memory of
size (not just a single element) which would be aligned.

Is this possible using
? The reason I ask is since
moves the pointer, the class using that pointer will give the allocator a pointer to the moved address for deallocation which would be invalid. Is there a way to create an aligned_allocator this way?


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
    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
} // 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 {
    Block(Block const&) = delete;
    Block& operator=(Block const&) = delete;

    explicit Block(size_t n);

    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();

} // Block<T>::~Block