Is casting and accessing an array of correct alignment and size to a not constructed Trivial Type undefined behaviour?

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Is the following code well defined?

struct S { int x; };
alignas(alignof(S)) char c_arr[sizeof(S)];
S *s_ptr = (S*)c_arr;
s_ptr->x = 5; // UB or not UB?

Note: S is purposfully defined as a Trivial Type.

Would the situation change if we made the type none trivial by adding a constructor that just sets X to some arbitrary value without calling it through placement new?

This question is different from this question because it does not use malloc and does ask also about none trivial types.

undefined-behavior c++
1 Answers

Is the following code well defined?

No. It violates the so called strict aliasing rule.

You must launder the pointer:

S *s_ptr = std::launder(reinterpret_cast<S*>(c_arr));
s_ptr->x = 5; // not UB

Prior to acceptance of proposal P0593R6, laundering would not have been sufficient. Instead, it used to be necessary to explicitly create the object using placement-new. That is still necessary when the reinterepreted type is not an "implicit-lifetime-type".

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