Is this a C++ alternative to the struct hack?

Question

Is the following valid C++? It's an alternative way of implementing a variable length tail to a flat structure. In C this is commonly done with the struct hack

struct Str
{
    Str(int c) : count(c) {}
    size_t count;
    Elem* data() { return (Elem*)(this + 1); }
};

Str* str = (Str*)new char[sizeof(Str) + sizeof(Elem) * count];
new (str) Str(count);
for (int i = 0; i < count; ++i)
    new (str->data() + i) Elem();
str->data()[0] = elem0;
str->data()[1] = elem1;
// etc...

I ask this in response to the following related question

Answer 1

No, it is not valid:

Elem might have different alignment than Str, so (reinterpret_)casting Str+1 to Elem* might or might not give you a valid pointer, and acccessing might give undefined behavior.

But after all, why would you want to do something like that?