Determining maximum possible alignment in C++

jalf_33213 0
Viewed 7631

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.

alignment c++11 c++
6 Answers

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.

user_1512920

This is what I'm using. In addition to this, if you're allocating memory then a new()'d array of char with length greater than or equal to max_alignment will be aligned to max_alignment so you can then use indexes into that array to get aligned addresses. 

enum {
            max_alignment = boost::mpl::deref<
                boost::mpl::max_element<
                        boost::mpl::vector<
                            boost::mpl::int_<boost::alignment_of<signed char>::value>::type,
                            boost::mpl::int_<boost::alignment_of<short int>::value>::type,
                            boost::mpl::int_<boost::alignment_of<int>::value>::type,                                boost::mpl::int_<boost::alignment_of<long int>::value>::type,
                            boost::mpl::int_<boost::alignment_of<float>::value>::type,
                            boost::mpl::int_<boost::alignment_of<double>::value>::type,
                            boost::mpl::int_<boost::alignment_of<long double>::value>::type,
                            boost::mpl::int_<boost::alignment_of<void*>::value>::type
                        >::type
                    >::type
                >::type::value
            };
        }
user_12820980
Related