Alignment guarantees of static char array

Question

I want to know the alignment guarantees of a statically allocated array of char. Looking at other SO questions, I found some concerning dynamically allocated ar

Answer 1

If you want to guarantee the alignment of the static char array, you can use the union trick.

union
{
    char buff[sizeof(T)];
    uint64_t dummy;
};

Here the alignment will be guaranteed for the largest element in the union. And of course you should wrap this nastiness away in a nice class.

Edit: better answer:

Of course you'd be even better using boost::aligned_storage or alignas for C++11.

Answer 2

No. Statically allocated arrays are aligned to sizeof(element_type) bytes -- for char it is 1 byte, which basically guarantees no alignment.

Answer 3

In C++11, the proper way to do that is this:

char alignas(T) buff[sizeof(T)]; //Notice 'alignas' as
T * pT = (T*) buff;
new(pT) T(); // well defined!

Notice the use of alignas.

If T is a template argument, then it is bettter to use std::alignment_of class template as:

char alignas(std::alignment_of<T>::value) buff[sizeof(T)]; 

Also note that the argument to alignas could be positive integral value Or type. So both of these are equivalent:

char alignas(T)          buff[sizeof(T)];
char alignas(alignof(T))  buff[sizeof(T)]; //same as above

The second one makes use of alignof which returns an integral value of type std::size_t.