Does using std::array as storage invoke UB?

Question

Is it okay to use std::array<std::byte, N> storage as storage for allocation of some memory? For example, is it safe to call placement-new on a.data(), even though the lifetime of the allocated object has nothing to do with std::array?

std::aligned_storage_t was deprecated in C++23 for a reason, that it has UB rooted in its design (and poor API but it's not the case in this question), while it's pretty much close to this use case of std::array, so is it mandatory to use C-style array std::byte storage[N] as a storage for memory allocation, not std::array (or maybe even std::vector).

Answer 1

The issue with std::aligned_storage_t is that when you start the lifetime of the new object, the lifetime of the std::aligned_storage_t object ends. [basic.life]p1:

The lifetime of an object o of type T ends when:

• [...]
• the storage which he object occupies [...] is reused by an object that is not nested within o.

So if you were to use the std::array as if it were std::aligned_storage_t, you would not be able to call any of the member functions of the array since the arrays lifetime would end:

alignas(T) std::array<std::byte, sizeof(T)> arr;
T* ptr = std::construct_at(reinterpret_cast<T*>(&arr), ...);
// Lifetime of `arr` ends
// arr.data();  // UB: lifetime of `std::array` has ended

However, since you use the .data() pointer of std::array, then the lifetime would not end because the member std::byte[N] would provide storage for the new object [intro.object]p3:

If a complete object is created in storage associated with another object e of type “array of N unsigned char” or of type “array of N std​::​byte” that array provides storage for the created object if [...]

And by [intro.object]p4:

An object a is nested within another object b if:

• a is a subobject of b, or
• b provides storage for a, or
• the exists an object c where a is nested within c, and c is nested within b.

So instantiate the second option with b = the member std::byte[N] of arr and c = the newly constructed object, and the third option with c = the member std::byte[N] and a = the newly constructed object and b = the std::array<std::byte, N> object. The newly constructed object is nested within the std::array.

So it can be used in the same way as if the object were std::byte[N] instead of std::array<std::byte, N> (namely to provide storage for a new object that is nested in the array/std::array):

alignas(T) std::array<std::byte, sizeof(T)> arr;
T* ptr = std::construct_at(reinterpret_cast<T*>(arr.data()), ...);
// Lifetime of `arr` continues: The array provides storage for the `T` object.
for (std::byte repr : arr) {
    // bytes of the object representation of `*ptr`
}
T* ptr2 = std::launder(reinterpret_cast<T*>(arr.data());  // Points to the same object as `*ptr`