Is it legal to alias a char array through a pointer to int?

Question

I know that the following is explicitly allowed in the standard:

int n = 0;
char *ptr = (char *) &n;
cout << *ptr;

What about this?

alignas(int) char storage[sizeof(int)];
int *ptr = (int *) &storage[0];
*ptr = 0;
cout << *ptr;

Essentially, I'm asking if the aliasing rules allow for a sequence of chars to be accessed through a pointer to another type. I'd like references to the portions of the standard that indicate one way or another if possible.

Some parts of the standard have left me conflicted; (3.10.10) seems to indicate it would be undefined behavior on the assumption that the dynamic type of storage is not int. However, the definition of dynamic type is unclear to me, and the existence of std::aligned_storage would lead me to believe that this is possible.

Answer 1

The code int *ptr = (int *) &storage[0]; *ptr = 0; causes undefined behaviour by violating the strict aliasing rule (C++14 [basic.lval]/10)

The objects being accessed have type char but the glvalue used for the access has type int.

The "dynamic type of the object" for a char is still char. (The dynamic type only differs from the static type in the case of a derived class). C++ does not have any equivalent of C's "effective type" either, which allows typed objects to be "created" by using the assignment operator into malloc'd space.

---

Regarding correct use of std::aligned_storage, you're supposed to then use placement-new to create an object in the storage. The use of placement-new is considered to end the lifetime of the char (or whatever) objects, and create a new object (of dynamic storage duration) of the specified type, re-using the same storage. Then there will be no strict aliasing violation.

You could do the same thing with the char array, e.g.:

alignas(int) char storage[sizeof(int)];
int *ptr = new(storage) int;
*ptr = 0;
cout << *ptr;

Note that no pseudo-destructor call or delete is required for built-in type int. You would need to do that if using a class type with non-trivial initialization. Link to further reading