Tag Archives: libsigc++

gtkmm 4 started

We (the gtkmm developers) have started work on an ABI-breaking gtkmm-4.0, as well as an ABI-breaking glibmm, target GTK+ 4, and letting us clean up some cruft that has gathered over the years. These install in parallel with the existing gtkmm-3.0 and glibmm-2.4 APIs/ABIs.

A couple of days ago I released first versions of glibmm (2.51.1) and gtkmm (3.89.1), as well as accompanying pangomm and atkmm releases.

This also lets us use my rewrite of libsigc++ for libsigc++-3.0, bringing us more fully into the world of “modern C++”. We might use this opportunity to make other fundamental changes, so now is the time to make suggestions.

We did a parallel-installing ABI-breaking glibmm even though glib isn’t doing that. That’s because, now that GTK+ is forcing us to do it for gtkmm, this seems like as good a time as any to do it for glibmm too. It’s generally harder to maintain C++ ABIs than C ABIs, largely because C++ is just more complicated and its types are more exactly specified. For instance we’d never have been able to switch to libsigc++-3.0 without breaking ABI.


C++17: constsexpr if

Yesterday I played with constexpr if a little, using clang++ built from the latest LLVM sources. C++17 will not have many of the features I was looking forward to, but this is a really nice convenience.

It lets us clean up some of those little templates-plus-specializations that only exist to execute different code in a parent template depending on the template’s type, particularly when the specialization is on a bool or integer value. It won’t help if the template specializations need different member variables or need to provide different type aliases or typedefs.

For instance, libsigc++-3.0 still had a little with_type template with partial template specializations for true and false. Replacing it with constexpr if makes the intention clearer.

I made many similar simplifications by using constexpr if in my murrayc-tuple-utils, which is used by libsigc++-3.0 via copy/paste/rename.

In my murrayc-dp-algorithms project, some code became much clearer and more efficient with constexpr if, allowing me to remove some template specializations that only existed to satisfy the compiler.

libsigc++ 2.99.5: Even less code

I managed to rip out another chunk of slightly-incomprehensible code from libsigc++, and those improvements are in libsigc++-3.0’s 2.99.5 version.

Now there’s no common functor_base base class and I removed all those interdependent result_type typedefs that were scattered throughout the code. When I had unravelled the whole result_type chain, I could replace it with just a handful of decltype(auto) uses, realising that that’s what the whole mess was trying to achieve. I love decltype(auto).

Trying CMake Again

I’ve recently been playing with CMake again, with much more success than last time. I still don’t love it, but:

I first tried it in the cmake branch of my little PrefixSuffix application because its build is rather simple.

I then finally allowed libsigc++ to have an additional CMake build system, at least in libsigc++-3.0, and Marcin Kolny got it done. This was possible now that libsigc++ doesn’t generate code from .m4 files. We wouldn’t have wanted to maintain two complicated build systems, but two simple build systems seems acceptable. I noticed many questions on StackExchange about actually using libsigc++ in CMake builds systems, so I added a hint about that to the documentation.

I also added a CMake build system to my Glom source code, to test something bigger, with many awkward dependencies such as Python, Boost, and PostgreSQL. It’s working well now, though I haven’t yet added all the tests to the build. This let me really try out CLion.

Minor Conclusions

I’m fairly pleased with the result. CMake doesn’t now feel massively more annoying than autotools, though it makes no attempt to do the dist and distcheck that I really need. I have now achieved a level of acceptance that means I wouldn’t mind much working on a project that uses CMake. I wouldn’t even need to hold my nose. The following complaints don’t seem to bother me quite so much any more:

I still find invoking CMake horribly obscure compared to ./configure. For instance

cmake -DCMAKE_INSTALL_PREFIX:PATH=/opt/something

seems ugly compared to

./configure --prefix=/opt/gnome

and I don’t see an equivalent for “./configure –help” to list all generic and project-specific build options. I also haven’t tried to replace mm-commons‘s wonderful¬† –enable-warnings=fatal to easily turn on compiler warnings as errors, encouraging my code to be free of warnings and deprecated API while not troubling people who just want to build the project with the least difficulties.

I still cannot understand how it’s normal in the CMake world to create these awful little Find*() scripts for each library that you might depend on, instead of just using pkg-config, though CMake’s pkg-config support seems to work well now. And hoping that your own library’s Find*() script ends up in CMake itself does not seem scalable.

It’s also a little odd that I cannot specify the link directories (-L flags) per target. link_directories() seems to apply to all targets.

CMake’s syntax still seems like a toy language. Its quoting (none/implicit/unknown) and separating (spaces, not commas) and use of whitespace (occasionally significant) sometimes makes m4 feel almost friendly. The non-case-sensitivity of function names (but case-sensitivity of variable names) just leads to arbitrarily inconsistent code and unnecessary style differences between projects.

I don’t like the CMake tradition of having separate CMakeList.txt files in sub-directories, because this can lead to duplication and makes it harder to see the whole build system at once. I prefer non-recursive autotools, so I tried to use just one CMakeList.txt for my CMake projects too, which was only a little awkward.

libsigc++: std::function-style syntax.

Yesterday I released version 2.99.2 of libsigc++-3.0. This changes the declaration syntax for sigc::slot and sigc::signal to use the same style as std::function, which was added in C++11. We don’t want to be arbitrarily and unnecessarily different.

We’ve also simplified sigc::mem_fun() to always take a reference, instead of either a reference or a pointer.

So, where you might do this for libsigc++-2.0:

sigc::slot<void, int, A> slot = sigc::mem_fun(this, &Thing::on_something);
signal<void, int, A> m_signal;

You would now do this for libsigc++-3.0.

sigc::slot<void(int, A)> slot = sigc::mem_fun(*this, &Thing::on_something);
signal<void(int, A)> m_signal;

Actually, as of version 2.9.1 of libsigc++-2.0, you can use both syntaxes with libsigc++-2.0, allowing you some time to adapt to the new API before one day moving to libsigc++-3.0.

libsigc++ 3.0: Very variadic

I have just released libsigc++ 2.99.1, the first release of the libsigc++-3.0 API, which installs in parallel with the existing libsigc++-2.0 API. This is libsigc++ using much more modern C++, specifically C++14. The API itself is almost completely unchanged, but the implementation is now clearer and easier to contribute to. I’m rather proud of my work, but I’m quite sure that there’s still room for improvement.

Unfortunately, glibmm and gtkmm can’t use libsigc++-3.0 until they have their own parallel-installable versions. That’s not likely to happen until the mythical GTK+ 4 happens. Even though libsigc++ is mostly all templates in headers, its symbols do appear in the linker symbols for gtkmm method signatures, so changes to libsigc++ can break gtkmm ABI. I do have a sigc3 branch of glibmm in git just to show that it works.

With around 150 commits, I gradually refactored libsigc++ to use variadic templates with C++14 instead of generating code from nasty m4 files. Over the years, we have built up quite a large set of regression tests, run during “make check” and “make distcheck”, including various corner cases. Without these tests, the refactoring would have been much harder. With the tests, I could make small commits, knowing that each commit had not broken anything. When something did seem wrong, I could add a test and go back through the git history to find the problem, sometimes splitting commits into even smaller changes. I did a lot of that, rebasing several times, sometimes stopping and starting again after help from Jonathan Wakeley and Marcin Kolny. Those tests give me much more confidence in the end result than I could have if I had chosen to just reimplement the entire API from scratch.

The API is almost all .h files and according to wc, there are 24,145 lines of code in those files in libsigc++ 2.7.1 (after make), and  6,507 in libsigc++ 2.99.1. So there is now only 27% as much code.

This is possible because:

  • C++ variadic templates allow us to have one class or function where we previously had to generate multiple versions for 1 to 6 function parameters, sometimes with additional versions for const and non-const parameters or const and non-const (and volatile and non-volatile) member function pointers.
  • decltype(auto) lets us avoid lots of templated type traits just to correctly specify the correct type for methods.
  • The standard C++ type traits, such as std::conditional<>, std::result_of<>, std::is_base_of<>, std::remove_volatile<> and std::is_const<> let us write very generic code, sometimes replacing our own type traits. I added some more to libsigc++ to get compile-time type traits for member method pointers.
  • template aliases (like typedefs for templates) avoided the need for multiple functor classes deriving from a common base, even though I ended up not needing most of these aliases either.
  • I replaced our sigc::ref() and sigc::reference_wrapper() with std::ref() and std::reference_wrapper<>. Presumably these share a common history.
  • I removed some configure checks and ifdef-ed workarounds for older MSVC++ and Sun Forte compilers. Hopefully they aren’t necessary now, but we will see.

For some adaptors¬† I used the tuple utilities that I’ve been working on recently, for instance in sigc::bind(). These are copied into the libsigc++ source code, and I’d particularly welcome improvements to them in the form of patches or github pull requests.

I’m still not completely happy with all the overloads we have for sigc::mem_fun(), to take member functions that are non-const, const, non-volatile and/or volatile, but I have some things still to try. We might also remove several by not allowing both mem_fun(pointer, func) and mem_fun(reference, func).

Please do suggest ways to simplify the code yet further.