c++ - Does the alignas specifier work with 'new'?

Question

My question is rather simple;

Does the alignas specifier work with 'new'? That is, if a struct is defined to be aligned, will it be aligned when allocated with new?

Answer 1

No it does not. The struct will be padded to the alignment requested, but it will not be aligned. There is a chance, however, that this will be allowed in C++17 (the fact that this C++17 proposal exists should be pretty good proof this can't work in C++11).

I have seen this appear to work with some memory allocators, but that was pure luck. For instance, some memory allocators will align their memory allocations to powers of 2 of the requested size (up to 4KB) as an optimization for the allocator (reduce memory fragmentation, possibly make it easier to reuse previously freed memory, etc...). However, the new/malloc implementations that are included in the OS X 10.7 and CentOS 6 systems that I tested do not do this and fail with the following code:

#include <stdlib.h>
#include <assert.h>

struct alignas(8)   test_struct_8   { char data; };
struct alignas(16)  test_struct_16  { char data; };
struct alignas(32)  test_struct_32  { char data; };
struct alignas(64)  test_struct_64  { char data; };
struct alignas(128) test_struct_128 { char data; };
struct alignas(256) test_struct_256 { char data; };
struct alignas(512) test_struct_512 { char data; };

int main() {
   test_struct_8   *heap_8   = new test_struct_8;
   test_struct_16  *heap_16  = new test_struct_16;
   test_struct_32  *heap_32  = new test_struct_32;
   test_struct_64  *heap_64  = new test_struct_64;
   test_struct_128 *heap_128 = new test_struct_128;
   test_struct_256 *heap_256 = new test_struct_256;
   test_struct_512 *heap_512 = new test_struct_512;

#define IS_ALIGNED(addr,size)   ((((size_t)(addr)) % (size)) == 0)

   assert(IS_ALIGNED(heap_8, 8));
   assert(IS_ALIGNED(heap_16, 16));
   assert(IS_ALIGNED(heap_32, 32));
   assert(IS_ALIGNED(heap_64, 64));
   assert(IS_ALIGNED(heap_128, 128));
   assert(IS_ALIGNED(heap_256, 256));
   assert(IS_ALIGNED(heap_512, 512));

   delete heap_8;
   delete heap_16;
   delete heap_32;
   delete heap_64;
   delete heap_128;
   delete heap_256;
   delete heap_512;

return 0;
}