Can `reinterpret_cast` Create Objects with Trivial Default Constructors?

Objects with Trivial Default Constructors: Can You Create Them with reinterpret_cast?

A recent update to cppreference's documentation has raised questions about the validity of using reinterpret_cast to create objects with trivial default constructors. Let's delve into the issue and examine the key concepts involved.

Can reinterpret_cast Create Objects?

The initial quote suggested that reinterpret_cast could be used to create objects with trivial default constructors by casting suitably aligned memory. However, subsequent changes to the documentation refute this claim.

Lifetime and Ownership

Even if reinterpret_cast could create objects, it's crucial to understand that it does not initialize or grant ownership of the object. As per the C standard, objects can only be created through specific mechanisms such as definitions, new-expressions, and union conversions.

Undefined Behavior

Attempting to access or modify an object that does not exist results in undefined behavior. In the example provided:

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

There is no X object in memory, so accessing its members via the reinterpret_cast pointer will lead to unpredictable and potentially dangerous consequences.

Conclusion

Using reinterpret_cast to create objects with trivial default constructors is not a valid or reliable practice. Objects must be explicitly created through proper mechanisms to ensure proper initialization, lifetime management, and well-defined behavior.

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