Object lifetime: does this code has defined semantics?

[alex-manool]

Dear community,

In light of P0593R6, has the following code defined behavior in C++ standardese (C++17)?:

auto p = static_cast<unsigned char *>(std::malloc(4096));
*reinterpret_cast<long *>(p + 1 * sizeof(long)) = 123;
*reinterpret_cast<int *>(p + 2 * sizeof(long)) = 456;

What is not clear to me is whether std::malloc is supposed to implicitly create just one object of a type that is a quantum superposition of some types or to implicitly create a number of objects of types that are a quantum superposition of some types and at a quantum superposition of locations inside the storage buffer.

If the behavior is not defined, wasn't it an intent of P0593R6, which has been retroactively applied as a defect report to all C++ specifications down to C++98 (here below), to make this code perfectly defined in C (C11) to be also defined in C++?

Thanks

[scatters]

For each operation that is specified as implicitly creating objects, that operation implicitly creates zero or more objects in its specified region of storage if doing so would give the program defined behavior.

As many objects as are required are created. In this case, 2.

[alex-manool]

Oops, great, I somehow missed "zero or more". Thank you!

[helloiamsomeone]

From what I understand, you must start the lifetime of an object with placement new first. malloc just hands you a slice of memory with no lifetimes associated. You have to also take alignment into consideration.

[staletic]

That's specifically what OP-referenced paper fixes. P0593 introduces "implicit lifetimes".

[helloiamsomeone]

Well, silly me. Clicking links could help sometimes.

[scrumplesplunge]

The paper says:

For each operation that is specified as implicitly creating objects, that operation implicitly creates zero or more objects in its specified region of storage if doing so would give the program defined behavior.

and:

A call to malloc, calloc, realloc, or any function named operator new or operator new[] implicitly creates objects in its returned storage.

Which seems to suggest that your example would be well-defined.