an aligned allocator.

aligned_allocator.h

#pragma once

#include <memory_resource>

inline void* _aligned_malloc(size_t size, size_t alignment)
{
    void* res = 0;
    void* ptr = malloc(size + alignment);
    if (ptr != 0)
    {
        res = reinterpret_cast<void*>((reinterpret_cast<size_t>(ptr) & ~(size_t(alignment - 1))) + alignment);
        *(reinterpret_cast<void**>(res) - 1) = ptr;
    }
    return res;
}

inline void* aligned_malloc(size_t size, size_t alignment)
{
#if MALLOC_ALREADY_ALIGNED
    return malloc(size);
#elif HAS_MM_MALLOC
    return _mm_malloc(size, alignment);
#elif HAS_POSIX_MEMALIGN
    void* res;
    const int failed = posix_memalign(&res, size, alignment);
    if (failed) res = 0;
    return res;
#elif (defined _MSC_VER)
    return _aligned_malloc(size, alignment);
#else
    return detail::_aligned_malloc(size, alignment);
#endif
}

inline void _aligned_free(void* ptr)
{
    if (ptr != 0)
        free(*(reinterpret_cast<void**>(ptr) - 1));
}

inline void aligned_free(void* ptr)
{
#if MALLOC_ALREADY_ALIGNED
    free(ptr);
#elif HAS_MM_MALLOC
    _mm_free(ptr);
#elif HAS_POSIX_MEMALIGN
    free(ptr);
#elif defined(_MSC_VER)
    _aligned_free(ptr);
#else
    detail::_aligned_free(ptr);
#endif
}

template <class T, int N>
class aligned_allocator
{
public:
    typedef T value_type;
    typedef T& reference;
    typedef const T& const_reference;
    typedef T* pointer;
    typedef const T* const_pointer;
    typedef size_t size_type;
    typedef ptrdiff_t difference_type;

    template <class U>
    struct rebind
    {
        typedef aligned_allocator<U, N> other;
    };

    inline aligned_allocator() throw() {}
    inline aligned_allocator(const aligned_allocator&) throw() {}

    template <class U>
    inline aligned_allocator(const aligned_allocator<U, N>&) throw() {}

    inline ~aligned_allocator() throw() {}

    inline pointer address(reference r) { return &r; }
    inline const_pointer address(const_reference r) const { return &r; }

    pointer allocate(size_type n, typename std::allocator<void>::const_pointer hint = 0);
    inline void deallocate(pointer p, size_type);

    inline void construct(pointer p, const_reference value) { new (p) value_type(value); }
    inline void destroy(pointer p) { p->~value_type(); }

    inline size_type max_size() const throw() { return size_type(-1) / sizeof(T); }

    inline bool operator==(const aligned_allocator&) { return true; }
    inline bool operator!=(const aligned_allocator& rhs) { return !operator==(rhs); }
};

template <class T, int N>
typename aligned_allocator<T, N>::pointer
aligned_allocator<T, N>::allocate(size_type n, typename std::allocator<void>::const_pointer hint)
{
    pointer res = reinterpret_cast<pointer>(aligned_malloc(sizeof(T)*n, N));
    if (res == 0)
        throw std::bad_alloc();
    return res;
}

template <class T, int N>
void
aligned_allocator<T, N>::deallocate(pointer p, size_type)
{
    aligned_free(p);
}