When Must Placement-New Be Used with `reinterpret_cast` to Create Trivially Default-Constructible Objects?

Reinterpret_cast and the Creation of Trivially Default-Constructible Objects

cppreference used to claim that reinterpret_cast could be used to create trivially default-constructible objects on suitably aligned storage. However, this claim has since been corrected to state that placement-new is required in such cases to formally introduce an object and avoid undefined behavior.

When Does Object Lifetime Begin?

Given this correction, it is clear that no X object exists in the code provided. Therefore, any attempt to access its members (e.g., reinterpret_cast>(buffer)->x = 42;) results in undefined behavior.

The Role of alignas()

The alignas() specifier ensures that the storage is suitably aligned to hold an X. It does not create an X object. If there were a branch between the alignas() declaration and the reinterpret_cast, it would not change the fact that no X object exists.

Changes in C Standards

The interpretation of this behavior has remained consistent between C 11 and C 1z. The concept of creating objects without explicit construction through reinterpret_cast is not supported in either standard.

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