Thoroughly Understanding C++ ABI

Not only, there is also XPC and IO/DriverKit on Apple, Android IPC, D-BUS, FIDL on Fucshia,....

Also C ABI also does not exist, people keep mistaking the ABI of their favourite C compiler with the OS ABI, which only overlap if the OS was written in C to start with.

For example on mainframes and micros, naturally not written in C, it is either a bytecode based ABI like TIMI on IBM i, or language environments like on z/OS, ClearPath MCP, OS 2200 and so forth.

And as a reminder, from a famous WG14 and WG21 member, and former Rust contributor,

"To Save C, We Must Save ABI"

https://thephd.dev/to-save-c-we-must-save-abi-fixing-c-funct...

And COM enforces lifecycle rules, transparent remoting, uniform activation (constructors basically), security primitives, marshalling, transparent async calls (yes, you can call any method asynchronously), and a ton of other things. It's actually very good and it's a shame most don't know about it and most of the rest sneer at it.

COM designers were inspired by C++ but their goals were different. They were designing a language-neutral binary standard (aka ABI) and hence took C++'s approach and tweaked it for their specification.

Here is an earlier comment of mine with some details - https://news.ycombinator.com/item?id=49142987

Given OOP, the only ABI for an object is a vtable and that is left unspecified (for good reason) by the C++ standard.

COM said, "objects are cool and modules are cool so we need an ABI for objects" and specified a vtable. It isn't the only programming model which continued from the same observation: Python did too with its absolutely abysmal leaky PyObject ABI

I understand that Move Semantics led to better implementations and that this is why things changed. I know that in this case, problems arose when transitioning from C++98 to what's commonly called Modern C++ (C++11). However, I'm not very familiar with what the specific issues were with COW. If you happen to know of any documents that describe this problem in more detail, I'd appreciate it if you could let me know. I'm sorry for always asking questions. I've looked up a few documents, but since you clearly know much more about this than I do, I'm asking to study it myself as well. If it's too much trouble, feel free not to reply.

No, ABI is not part of the C++ standard, although there are parts of C++ that do go "this is an ABI thing" (e.g., [[no_unique_address]]).

ABI is more of OS-level thing. Most systems these days follow the SysV ABI, which is largely defined by the hardware manufacturers via the processor-specific supplements (the x86-64 one is here: https://gitlab.com/x86-psABIs/x86-64-ABI). These largely delegate C++-specific conventions to the Itanium C++ ABI (which they likely directly reference), although the ARM ecosystem uses a somewhat different layout for the exception handling tables. Microsoft uses a different ABI for both the underlying C ABI and for the C++ compatibility layer built on top of the C ABI.

One of the issues that crops up is that vendors end up needing to add extensions to the ABI for various reasons, and these extensions tend to end up being incompatible, since they're added before they've had a time to be standardized. 16-bit floats is a particular historical bugbear, as is the C23 _BitInt stuff.

No, neither does the C standard.

With exception of Swift, D, and bytecode based languages, the ABI is left to the vendors.

For those that think ISO/IEC 9899:2024 PDF has anything related to ABI, the actual ABI used by C compilers, is the OS ABI, if the OS was written in C to start with, and naturally this overlaps quite nicely with UNIX like OSes, and Windows.

It isn't like that on other platforms that decided to either use other systems languages, or expose their OS APIs in a different way, e.g. mainframes, micros, Android, ChromeOS, WebOS,...

The C++ standard doesn't say anything about binary ABI (as you say generally either Itanium or MSVC, though there are other options they are rare). However the people who write that standard are very sensitive to the vendors and users of C++ who depend on a stable binary ABI. Thus they take extreme care to ensure that no change to the standard breaks binary ABI.

The C++ standard did force gcc to break the ABI of std::string (copy on write was banned - for good reason). They then watched the gcc community work through 10 years of pain to make the transition. They are also well aware that python 3 broke compatibility with Python 2 - and again it resulted in 10 years of pain for the python community to deal with that. With this history there are a lot of experts strongly against any breaking change. It might happen anyway, but only with strong justification and likely an attempt at a migration of some form (what? there are a lot of examples of migration plans that don't work that they are aware of)

Again, it is not because of convention, it is because of painful experience from those who break it.

It doesn't, and FWIW the article says that. A meta note: ABI stability is a holy war inside the C++ community. Those against it argue that it's holding back real language evolution. Herb Sutter is even working on his own C++ offshoot that shows what could be.

Neither of the ISO C and ISO C++ standards define an ABI.