deprecation of the stream_to constructor

[belst]

Hi,

I wanted to update the library to the latest version, but now I get deprecation warnings when using the stream_to constructor.

The static factory methods do not really work for me, since stream_to is not movable, so I cannot store it in an optional.

I guess I could write a wrapper like this:

struct stream_to_wrapper {
    stream_to_wrapper(
        transaction_base& tx,
        table_path path,
        std::initializer_list<std::string_view> columns = {}
    ) : to(stream_to::table(tx, path, columns)) {}

    stream_to to;
};

// Use like:
std::optional<stream_to_wrapper> s;
s.emplace(tx, table, columns);

(EDIT: this wrapper unfortunately does not work)

I have a database class which can only ever have 1 writer active, so I store it in an optional and keep writing incoming data until I manually commit it.

Is there a way to store the writer in an optional without using a wrapper struct?

[jtv]

ISTM we could make stream_to movable.

[jtv]

It's not as easy as it seems. We need a pointer to its transaction_focus sub-object, and that pointer will break if we move the stream.

But perhaps with some trickery in the move constructor...

[jtv]

I think I've cracked it. @belst could you try it with the branch in #707?

[belst]

Just replacing my current pointer based setup with optional::emplace seems to work. Thanks. Haven't done many more tests yet tho.

[jtv]

Please throw any testing at it that you can. I'm still a bit nervous about the way I had to std::move() an object just to get the base-class object, and then std::move() some of its members as well. A fun little puzzle.

[jtv]

A next step could be to support moving of more transaction_focus-derived classes.

In my test I used std::in_place for constructing the std::optional. I'll keep this project open for a while, so I can incorporate any further notes you may have @belst.

[jtv]

OK, it looks pretty solid to me, it's got a test, and I've now expressly blocked any move constructors in the hierarchy that I didn't want (previously blocked by transaction_focus base class not having one). I'll merge the PR.