Can `reinterpret_cast` Create Trivially Default-Constructible Objects in C ?

Reinterpret_Cast and Trivially Default-Constructible Objects

The C reference states that objects with trivial default constructors can be created using reinterpret_cast on suitably aligned storage. However, this statement has been revised and now reads differently.

Current Understanding:

Objects with trivial default constructors cannot be created directly using reinterpret_cast on allocated memory. Instead, placement-new must be used to formally initialize the object and avoid undefined behavior.

Reasons Behind the Revision:

The original statement implied the existence of an object on the allocated storage, which is incorrect. The creation of an object requires explicit language constructs such as a definition, new-expression, union member change, or temporary object creation.

The definition of "object" in C 1z (intro.object/1) explicitly states that objects are created by these specific mechanisms, and does not include reinterpret_cast.

Lifetime of the X Object:

Since no X object is created, the concept of its lifetime does not apply. The behavior of assigning to its member variable (x) results in undefined behavior.

Changes in C 1z:

C 1z has changed the definition of "object" and clarified that reinterpret_cast alone does not create objects. This reinforces the principle that objects must be explicitly created and initialized through well-defined language mechanisms.

Conclusion:

While reinterpret_cast can be used to access the memory of uninitialized storage, it cannot create objects with trivial default constructors directly. Proper object initialization and lifetime management require the use of appropriate language constructs such as placement-new or explicit constructor calls.

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