When Does `reinterpret_cast` Actually Create Objects?

Reinterpret_cast and Trivial Default Constructors: A Journey into Undefined Behavior

The cppreference documentation once claimed that trivial default-constructible objects could be created using reinterpret_cast on suitably aligned memory. However, this assertion has since been revised to emphasize the requirement of using placement-new to explicitly initialize such objects.

The Mystery of Lifetime

If reinterpret_cast did create objects, we would encounter a dilemma regarding their lifetime. Would the casting operation itself mark the object's inception? Or would it occur earlier, when the storage was allocated with an expression like alignas(X) char buffer[sizeof(X)]?

In the latter case, it raises the intriguing possibility that a conditional statement between storage allocation and casting could potentially construct either an X object or some other trivially default-constructible entity, Y.

The Clarifying Guidance of the Standard

Unfortunately, the C standard explicitly excludes such amorphous object creation. According to [intro.object]/1, objects can only come into being through a definition, a new-expression, the activation of a union's active member, or the creation of a temporary object.

Not Even a Vacuous Existence

The notion of a vacuous initialization lifetime is irrelevant to this situation. For that concept to apply, an object must first exist, which is not the case here.

Consistently Undefined

Throughout the history of C , the behavior of reinterpret_casting trivially default-constructible objects has remained consistently undefined. Both C 11 and C 1z adhere to the fundamental principle that distinct objects must have distinct addresses, rendering the creation of multiple objects within the same storage a Schrödinger's cat-like paradox.

Conclusion

The reinterpret_cast operation is not a magical tool for creating objects. In attempting to invoke its creative powers, programmers risk invoking undefined behavior and plunging their code into the treacherous depths of unpredictability.

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