best cross-platform method to get aligned memory

Question

Here is the code I normally use to get aligned memory with Visual Studio and GCC

inline void* aligned_malloc(size_t size, size_t align) {
    void *result;
          


        

Answer 1

If you compiler supports it, C++11 adds a std::align function to do runtime pointer alignment. You could implement your own malloc/free like this (untested):

template
void *aligned_malloc(std::size_t size)
{
    std::size_t space = size + (Align - 1);
    void *ptr = malloc(space + sizeof(void*));
    void *original_ptr = ptr;

    char *ptr_bytes = static_cast(ptr);
    ptr_bytes += sizeof(void*);
    ptr = static_cast(ptr_bytes);

    ptr = std::align(Align, size, ptr, space);

    ptr_bytes = static_cast(ptr);
    ptr_bytes -= sizeof(void*);
    std::memcpy(ptr_bytes, original_ptr, sizeof(void*));

    return ptr;
}

void aligned_free(void* ptr)
{
    void *ptr_bytes = static_cast(ptr);
    ptr_bytes -= sizeof(void*);

    void *original_ptr;
    std::memcpy(&original_ptr, ptr_bytes, sizeof(void*));

    std::free(original_ptr);
}

Then you don't have to keep the original pointer value around to free it. Whether this is 100% portable I'm not sure, but I hope someone will correct me if not!