c++ - 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 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

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.