The fact C++ has no stable ABI is not what stops you from creating language bindings, because you need that kind of interoperability only for those symbols that need to cross the FFI.
It's more about how the library is designed than the implementation language itself; for instance, Tensorflow is mostly written in C++, but its intended use is as a Python module. Also, by using GLib you take in so many GLib-specific types and assumptions it becomes extremely wonky using it from non-GLib code, and you often end up writing some kind of FFI anyway in order to wrap its behaviour. I've even seen some libraries ship both a C and a GLib wrapper, as if it was almost like a separate language.
LLVM is a good example of C++ being able to interface with other languages using its C FFI, being completely written in C++ but having bindings for almost every programming language.
I can count among them at least C, C++, D, Rust, OCaml and Python, for instance, and while it has an "LLVM-C" library available, you are not necessarily forced to use it; you can just export what you need directly from C++ using a file with a few functions defined as extern "C".
Rust is probably even trickier to interface with C but it's still pretty doable, given that GNOME is now rewriting several components in Rust (see librsvg).
I still think one of the main reasons C++ was discarded so quickly at the time, despite Qt and KDE being older than GNOME and in C++ already, was being born from a culture that had at the time already rejected the adoption of C++.
GNU has historically always discouraged its projects, especially those they cared about the most, from using C++. It's well documented how strongly RMS despised the language and how he actively pushed people towards using C for such tasks. I think it's still written in somewhere in the GNU website, even.
Heck the GCC developers had to ask RMS the permission to switch to C++ a few years ago, due to GCC becoming harder and harder to maintain. Ironically, it used internally its own private C implementation of dynamic vectors, while being one of the main implementers of the STL. I think it even had some sort of garbage collection inside, and that they largely reimplemented an object system too using C and macros (it's been a while since I read the GCC sources, so my memories could be incorrect). RMS wasn't really that happy about it, but it grumpily said they could.
LLVM is a good example of C++ being able to interface with other languages using its C FFI, being completely written in C++ but having bindings for almost every programming language. I can count among them at least C, C++, D, Rust, OCaml and Python, for instance, and while it has an "LLVM-C" library available, you are not necessarily forced to use it; you can just export what you need directly from C++ using a file with a few functions defined as extern "C".
Rust is probably even trickier to interface with C but it's still pretty doable, given that GNOME is now rewriting several components in Rust (see librsvg).
I still think one of the main reasons C++ was discarded so quickly at the time, despite Qt and KDE being older than GNOME and in C++ already, was being born from a culture that had at the time already rejected the adoption of C++.
GNU has historically always discouraged its projects, especially those they cared about the most, from using C++. It's well documented how strongly RMS despised the language and how he actively pushed people towards using C for such tasks. I think it's still written in somewhere in the GNU website, even.
Heck the GCC developers had to ask RMS the permission to switch to C++ a few years ago, due to GCC becoming harder and harder to maintain. Ironically, it used internally its own private C implementation of dynamic vectors, while being one of the main implementers of the STL. I think it even had some sort of garbage collection inside, and that they largely reimplemented an object system too using C and macros (it's been a while since I read the GCC sources, so my memories could be incorrect). RMS wasn't really that happy about it, but it grumpily said they could.