libpqxx 7.7.0: lots of new features, lots of fixes.

It's going to be a big one. Believe me, I toyed with the idea of calling it 8.0. But I think it's not quite radical enough for that. I'm still hoping to raise the language baseline to C++20 for libpqxx 8.0. We'll see.

But here's what you get:

This version introduces a new class, connecting, for connecting to the database asynchronously. It's not native C++ async support, and it's not based on coroutines, but you can build coroutines on top of it. Kirit Sælensminde has built a sample project for this..

You can now convert SQL range values to and from C++ strings, using the new pqxx::range class.

There is now also support for converting between SQL timestamps (without time zones) and C++ std::chrono::year_month_day values. It's pretty limited, because there are lots of inherent problems with date/time support. For one, it only works with the ISO date format.

If you want fast access to a result's fields without having to go through row objects, the pqxx::result class now has a two-dimensional at() member that takes a row number and a column number. And since there's a proposal for permitting two-dimensional array indexing in C++23, there's a provisional two-dimensional operator[] as well. (However I can't test this yet, so it could be broken.)

The project now has a file requirements.json which lists the minimum major versions of compilers, C++, and PostgreSQL that you need for the current version of libpqxx. It's not going to be too detailed, because the more detail I put in there, the more is going to go wrong in maintenance. But at least it should be easy to see in the future which libpqxx versions require which C++ versions etc.

The Doxygen documentation now uses the (Doxygen-extended) Markdown format. That should make the documentation comments in the libpqxx headers a little more modern and easier to read.

Also, we now build docs in doc/html/, no longer in doc/html/Reference/.

There are also some changes that may affect you without exactly creating serious incompatibilities. More functions are [[nodiscard]]. Some member functions are now lvalue-qualified or rvalue-qualified. You can't call an lvalue-qualified function on an rvalue object — so I lvalue-qualified some functions that sane code would not call as the last operation on an object. I also rvalue-qualified one member function that effectively invalidates its object.

Finally, I promised you a lot of fixes:

• Fix stream_to for differing table/client encodings. (#473)
• Use [[likely]] & [[unlikely]] only in C++20, to silence warnings.
• Fix clang "not a const expression" error in Windows. (#472)
• Fix warnings about [[likely]] in if constexpr. (#475)
• Clearer error for ambiguous string conversion of char type. (#481)
• Statement name in prepare() error was for the wrong statement. (#488)
• Helper for implementing string traits: generic_to_buf.
• Work around broken std::filesystem::path in MinGW. (#498)
• Fix leak when getting client encoding fails. (#500)
• Re-enable pyflakes testing in tools/lint.
• Don't run clang-tidy by default. Compatibility issues with gcc options.
• Disable some std::filesystem features on Windows.
• Shut up stupid Visual Studio warnings.
• On gcc, mark rarely-used functions as "cold," to be optimised for size.
• Glyph scanning for GB18030 encoding was utterly broken. (#517)

Did I promise too much? Hope you all have a great 2022!