Skip to content

ustring: TODO to avoid copies from operator string + C++20 move from outstreams

Daniel Boles requested to merge dboles/ustring-TODOs into master

I think these are worth doing, once ABI-break and C++20 come around respectively.

What do others think?

For operator std::string() is there any risk of breakage? I can't see how, if we overload on both const& and &&, unless someone currently captures the value from operator std::string() const by auto const& and extends the lifetime of that temporary, which won't happen once it returns a reference to the member. But such code is probably suspect anyway, isn't it? And I don't think is justification to keep silently copying on convert.

The move(stream).str() seems to be a no-brainer, clawing back a bit of performance from what is probably overall a bad picture due to using streams and having to copy again via GLib. Is there any way to avoid the latter, too?

commit 615efbd2cff735c0468a9e6e7d42af050a2af850 (HEAD -> dboles/ustring-TODOs, origin-ssh/dboles/ustring-TODOs)
Author: Daniel Boles <dboles.src@gmail.com>
Date:   Tue Dec 12 15:54:31 2023 +0000

    ustring: Add TODOs for C++20 move from outstreams
    
    C++20 adds `ostream.str() &&` for rvalue `this`, which will move out the
    stream's owned string. We can use that once we are on C++20 to avoid the
    extra copy of the string (although we do then copy that AGAIN via GLib!)

commit e3b91d509a1d51241d9aaae8a1bab0966adf2024
Author: Daniel Boles <dboles.src@gmail.com>
Date:   Tue Dec 12 15:50:28 2023 +0000

    ustring: TODO to avoid copies from operator string
    
    Our conversion operator to std::string always returns a copy but that is
    wasteful when it could be a const& reference or moved-out from an rvalue
    – especially bad since conversions can occur silently & harm performance
    without users realising. We can split to const&/&& overloads at next ABI
Edited by Daniel Boles

Merge request reports