Determining maximum possible alignment in C++
alignment, c++, c++11
Solution
In C++0x, the `Align` template parameter of `std::aligned_storage<Len, Align>` has a default argument of "default-alignment," which is defined as (N3225 §20.7.6.6 Table 56):
The value of default-alignment shall be the most stringent alignment requirement for any C++ object type whose size is no greater than `Len`.
It isn't clear whether SSE types would be considered "C++ object types."
The default argument wasn't part of the TR1 `aligned_storage`; it was added for C++0x.
Problem
Is there any portable way to determine what the maximum possible alignment for any type is? For example on x86, SSE instructions require 16-byte alignment, but as far as I'm aware, no instructions require more than that, so any type can be safely stored into a 16-byte aligned buffer. I need to create a buffer (such as a char array) where I can write objects of arbitrary types, and so I need to be able to rely on the beginning of the buffer to be aligned. If all else fails, I know that allocating a char array with `new` is guaranteed to have maximum alignment, but with the TR1/C++0x templates `alignment_of` and `aligned_storage`, I am wondering if it would be possible to create the buffer in-place in my buffer class, rather than requiring the extra pointer indirection of a dynamically allocated array. Ideas? I realize there are plenty of options for determining the max alignment for a bounded set of types: A union, or just `alignment_of` from TR1, but my problem is that the set of types is unbounded. I don't know in advance which objects must be stored into the buffer.