How do you actually take a value out of optional? Meaning take ownership of the value inside the std::optional and replace it with std::nullopt (or swap it with another value)?
In Rust for example you could .unwrap your Option or do something like foo.take().unwrap(). I'm trying to do something like that with C++ optionals.
operator*/value() returns a reference to the value held by the optional, so you can simply use std::move to move it to a temporary variable
std::optional<std::string> opt = "abc";
// "take" the contained value by calling operator* on a rvalue to optional
auto taken = *std::move(opt);
This will invoke the rvalue reference overload of operator*() of the optional, which returns an rvalue reference to the contained value.
You can also directly perform std::move on the return value of the operator*() of the lvalue optional, which will convert the lvalue reference of the contained value into an rvalue
auto taken = std::move(*opt);
The standard way in C++ to replace a value and return the old, like mem::replace in Rust, is std::exchange.
So:
std::exchange(o, std::nullopt).value();