How does copy assignment work in for folly::Optional?

Question

I was reading the code in folly::Optional for copy assignment and I am not clear how exactly the call to construct() assigns a value to the optional. Specifically in construct() how does this expression work?

new (const_cast<void*>(ptr)) Value(std::forward<Args>(args)...);

You've linked an internal FB page there. Probably want to find the external link? — Mike Vine, Nov 09 '20 at 13:14
The link you gave is not for the copy constructor, and the code you quote isn't there, BTW. — Ami Tavory, Nov 09 '20 at 13:20
You may be interested in: https://en.cppreference.com/w/cpp/memory/construct_at. Folly seemingly just provided its custom version for pre-C++20 code (which is basically all code now ;). — Daniel Langr, Nov 09 '20 at 13:52
Edited the link to point to the external, openly available, version of folly::Optional. — Sid, Nov 11 '20 at 16:00

score 3 · Accepted Answer · answered Nov 09 '20 at 13:25

3

To deconstruct the line you wrote:

std::forward<Args>(args)... is performing variadic-template perfect forwarding. In essence, it means that whatever was an r-value will be forwarded to, and so on, for any number of arguments.

Value(std::forward<Args>(args)...) is calling the constructor of Value on these arguments.

new (const_cast<void*>(ptr)) ... is calling placement new.

So what the line is saying is, create an object at this specific memory location, constructing the object there by forwarding all the arguments you got.

answered Nov 09 '20 at 13:25

Ami Tavory

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I am a bit surprused by the lack of scope operator on the new. I guess that is too paranoid for FB. – Yakk - Adam Nevraumont Nov 09 '20 at 13:30
@Yakk-AdamNevraumont Interesting – Ami Tavory Nov 09 '20 at 13:37
@Yakk-AdamNevraumont In my experience, Facebook/Folly does not care much about portability and "corner cases". (For instance, its `PackedSyncPtr` works only on some listed architectures and simply triggers a compilation error on others.) – Daniel Langr Nov 09 '20 at 14:03

How does copy assignment work in for folly::Optional?

1 Answers1