When Does `reinterpret_cast` Create Objects in C and What are the Implications for Object Lifetime?

Reinterpret_cast and Default Constructor Creation

In C , certain objects can be conveniently created using reinterpret_cast on suitably aligned memory. However, the behavior and implications of such operations require clarification.

The original cppreference statement suggests that objects with trivial default constructors can be created using reinterpret_cast, implying that the following code is valid:

struct X { int x; };
alignas(X) char buffer[sizeof(X)];
reinterpret_cast<X*>(buffer)->x = 42;

However, a newer wording of the cppreference statement emphasizes that objects cannot be simply created by reinterpreting storage, and placement-new is necessary to avoid undefined behavior. This raises several questions:

1. Is the initial statement incorrect?
2. If it is correct, when does the lifetime of the X object begin, and does the cast itself constitute storage acquisition?
3. Are there any changes in this regard between C 11 and C 1z?

Correctness of the Initial Statement

The updated statement on cppreference is accurate. Unlike in C, objects cannot be created in C by simply reinterpreting suitable memory. According to the language standard, an object is created through explicit mechanisms like definitions, new-expressions, or when implicitly changing the active member of a union.

In the given code snippet, there is no object creation happening. While memory is allocated for a potential X object, no object is formally introduced into the code. Pretending that an X object exists based on this memory results in undefined behavior.

Lifetime and Storage Acquisition

The placement of the alignas specifier before the buffer declaration ensures proper alignment for an X object. However, the allocation of memory does not initiate the lifetime of an object.

alignas(X) char buffer[sizeof(X)]; // (A)

Similarly, the reinterpret_cast does not acquire storage for the X object either.

reinterpret_cast<X*>(buffer)->x = 42; // (B)

Since no object is created, there is no lifetime to consider, and the cast is not considered storage acquisition.

Changes from C 11 to C 1z

The interpretation of the quoted text remains the same between C 11 and C 1z. In C 11, the core paragraph regarding object creation is similar to the one introduced in C 1z. Although this particular paragraph is now considered the definition of "object" in C 1z, its interpretation does not change.

The original cppreference statement implying object creation through reinterpret_cast could potentially lead to misconceptions and misunderstandings of object creation rules in C . The updated wording accurately reflects the true nature of this operation and aligns with the language standard, which explicitly defines when an object is created and how its lifetime begins.

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