Does the alignas specifier work with 'new'

问题:

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?

回答1:

If your type's alignment is not over-aligned, then yes, the default new will work. "Over-aligned" means that the alignment you specify in alignas is greater than alignof(std::max_align_t). The default new will work with non-over-aligned types more or less by accident; the default memory allocator will always allocate memory with an alignment equal to alignof(std::max_align_t).

If your type's alignment is over-aligned however, your out of luck. Neither the default new, nor any global new operator you write, will be able to even know the alignment required of the type, let alone allocate memory appropriate to it. The only way to help this case is to overload the class's operator new, which will be able to query the class's alignment with alignof.

Of course, this won't be useful if that class is used as the member of another class. Not unless that other class also overloads operator new. So something as simple as new pair<over_aligned, int>() won't work.

A proposal for C++17 (which has been accepted) adds support for dynamic allocation of over-aligned types, by having overloads of operator new/delete that take the alignof of the type being allocated. This will also support alignments of less than the max aligned type, so your memory allocator need not always return memory aligned to alignof(std::max_align_t).

That being said, compilers are not required to support over-aligned types at all.

回答2:

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;
}