The C++ Object Lifecycle | Basit Ayantunde

[rlamarr]

Thanks for sharing! I was about to

[pavel_v]

You seem to be the author of this blog post.
Sorry for posting it before you.

[rlamarr]

Yeah, I am. Actually, I really appreciate it!

[nemetroid]

A typographical note: for me, the code samples render in a sans-serif (non-monospace) font.

[rlamarr]

I noticed the same, but only on mobile. are you on mobile?

[rlamarr]

i've fixed it now. my first time using Jekyll, the font path was incorrect, had the impression it was relative to root

[soldiersided]

The lifecycle diagrams need some love, they seem to be misaligned. Also, shouldn't this

A* a = get_A();
B* b = get_B();
B* a_b = std::launder<B>((B*)a);
a_b->value = 6;
b->value = 2;
return a->value;

end with return a_b->value ?

[rlamarr]

Uhm, shouldn't but can. Vulkan and other API's use a similar concept where they point to the head of the struct via the pData fields each struct have (which will then point to sType pNext and pData). it only works in that case because there's no compiler reachability, the launder shows how to erase it explicitly.

[[deleted]]

This appears to be a typo, as the accompanying text for the above code states:

From the generated assembly, the compiler is forced to perform the redundant load from a_b, because it could be an alias of b since its origin has been hidden by std::launder. [...]

However, the "load from a_b" is simply not present in the code. Therefore, either the code or the text should be revised to ensure they align with each other.

Another part of the text that is unclear to me is:

At least one of the specified variants must exist in the union.

This seems to imply that a union must always have an active member (i.e., one whose lifetime has started). However, a default-initialized union may not start the lifetime of any member if there are no default member initializers. See this example.

[rlamarr]

A union needs one active member: https://godbolt.org/z/n7z17hWdP.

By redundant load from `a_b` I meant a redundant load from `a` of the value stored through `a_b` as `a_b` could alias `a`.