reinterpret_cast creating a trivially default-constructible object

Question

cppreference^† states that:

Objects with trivial default constructors can be created by using reinterpret_cast on any suitably aligned storage, e.g. on memory allocated with std::malloc.

This implies that the following is well-defined code:

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

Three questions follow:

1. Is that quote correct?
2. If yes, at what point does the lifetime of the X begin? If on line (B), is it the cast itself that is considered acquiring storage? If on line (A), what if there were a branch between (A) and (B) that would conditionally construct an X or some other pod, Y?
3. Does anything change between C++11 and C++1z in this regard?

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^†Note that this is an old link. The wording was changed in response to this question. It now reads:

Unlike in C, however, objects with trivial default constructors cannot be created by simply reinterpreting suitably aligned storage, such as memory allocated with std::malloc: placement-new is required to formally introduce a new object and avoid potential undefined behavior.

Answer 1

There is no X object, living or otherwise, so pretending that there is one results in undefined behavior.

[intro.object]/1 spells out exhaustively when objects are created:

An object is created by a definition ([basic.def]), by a new-expression ([expr.new]), when implicitly changing the active member of a union ([class.union]), or when a temporary object is created ([conv.rval], [class.temporary]).

With the adoption of P0137R1, this paragraph is the definition of the term "object".

Is there a definition of an X object? No. Is there a new-expression? No. Is there a union? No. Is there a language construct in your code that creates a temporary X object? No.

Whatever [basic.life] says about the lifetime of an object with vacuous initialization is irrelevant. For that to apply, you have to have an object in the first place. You don't.

C++11 has roughly the same paragraph, but doesn't use it as the definition of "object". Nonetheless, the interpretation is the same. The alternative interpretation - treating [basic.life] as creating an object as soon as suitable storage is obtained - means that you are creating Schrödinger's objects^*, which contradicts N3337 [intro.object]/6:

Two objects that are not bit-fields may have the same address if one is a subobject of the other, or if at least one is a base class subobject of zero size and they are of different types; otherwise, they shall have distinct addresses.

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_{^* Storage with the proper alignment and size for a type T is by definition storage with the proper alignment and size for every other type whose size and alignment requirements are equal to or less than those of T. Thus, that interpretation means that obtaining the storage simultaneously creates an infinite set of objects with different types in said storage, all having the same address.}

Answer 2

Based on p0593r6 I believe the code in the OP is valid and should be well defined. The new wording, based on the DR retroactively applied to all versions from C++98 inclusive, allows implicitly object creation as long as the created object is well defined (tautology is sometimes the rescue for complicated definitions), see § 6.7.2.11 Object model [intro.object]):

implicitly-created objects whose address is the address of the start of the region of storage, and produce a pointer value that points to that object, if that value would result in the program having defined behavior [...]

See also: https://stackoverflow.com/a/61999151/2085626

Answer 3

This analysis is based on n4567, and uses section numbers from it.

§5.2.10/7: When a prvalue v of object pointer type is converted to the object pointer type “pointer to cv T”, the result is static_cast<cv T*>(static_cast<cv void*>(v)).

So, in this case, the reinterpret_cast<X*>(buffer) is the same as static_cast<X *>(static_cast<void *>(buffer)). That leads us to look at the relevant parts about static_cast:

§5.2.9/13: A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T”, where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. The null pointer value is converted to the null pointer value of the destination type. If the original pointer value represents the address A of a byte in memory and A satisfies the alignment requirement of T, then the resulting pointer value represents the same address as the original pointer value, that is, A.

I believe that's enough to say that the original quote is sort of correct--this conversion gives defined results.

As to lifetime, it depends on what lifetime you're talking about. The cast creates a new object of pointer type--a temporary, which has a lifetime starting from the line where the cast is located, and ending whenever it goes out of scope. If you have two different conversions that happen conditionally, each pointer has a lifetime that starts from the location of the cast that created it.

Neither of these affects the lifetime of the object providing the underlying storage, which is still buffer, and has exactly the same lifetime, regardless of whether you create a pointer (of the same or converted type) to that storage or not.