char array as storage for placement new

Question

Is the following legal C++ with well-defined behaviour?

class my_class { ... };

int main()
{
    char storage[sizeof(my_class)];
    new ((void *)storage) m         


        

Answer 1

The char array may not be aligned correctly for the size of myclass. On some architectures, that means slower accesses, and on others, it means a crash. Instead of char, you should use a type whose alignment is equal to or greater than that of the struct, which is given by the largest alignment requirement of any of its members.

#include 

class my_class { int x; };

int main() {
    uint32_t storage[size];
    new(storage) my_class();
}

To allocate enough memory for one my_class instance, I think size ought to be sizeof(my_class) / sizeof(T), where T is whichever type you use to get the correct alignment.