How to align pointer

Question

How do I align a pointer to a 16 byte boundary?

I found this code, not sure if its correct

char* p= malloc(1024);

if ((((unsigned long) p) % 16) !=          


        

Answer 1

C++0x proposes std::align, which does just that.

// get some memory
T* const p = ...;
std::size_t const size = ...;

void* start = p;
std::size_t space = size;
void* aligned = std::align(16, 1024, p, space);
if(aligned == nullptr) {
    // failed to align
} else {
    // here, p is aligned to 16 and points to at least 1024 bytes of memory
    // also p == aligned
    // size - space is the amount of bytes used for alignment
}

which seems very low-level. I think

// also available in Boost flavour
using storage = std::aligned_storage_t<1024, 16>;
auto p = new storage;

also works. You can easily run afoul of aliasing rules though if you're not careful. If you had a precise scenario in mind (fit N objects of type T at a 16 byte boundary?) I think I could recommend something nicer.