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 arrays of char.

For statically allocated char arrays, are they aligned such that I can placement new any type into it (provided it is sufficiently large)? Or does this only apply for dynamically allocated ones?

char buff[sizeof(T)];
T * pT = (T*) buff;
new(pT) T(); // well defined?
...
pT->~T();

If not, how can I overcome this problem?

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.