C++ ABIs are why Win32 settled on things like COM to do cross-module passing of objects. Because nobody could agree on a standard, the COM standard enforced a very specific calling convention, and a very specific vtable layout for COM objects.
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,
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.
(Though you cannot call just any method asynchronously; the interface's IDL needs specific annotations and the underlying object needs to implement ICallFactory.)
COM was one of the best realizations of "Component-based Software Engineering" and Brad Cox's "Software-IC" model. It was a binary standard and so components could be written in any language and yet be assured of perfect interoperability provided you followed all the rules/conventions. There was a bunch of boiler-plate for the framework itself but once you understood it, everything was smooth sailing in your language of choice.
One of the things i always advise people is not to focus only on the current way of doing things but to study older well-known libraries/frameworks/architectures/kernels/etc. to really understand "Software Engineering" from many perspectives. That is where insight comes from and real understanding happens.
For people interested in understanding COM, see the classic Essential COM by Don Box.
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.
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.
The changes to std::string were made because "the [pre-C++11] definition of basic_string allows only very limited concurrent access to strings. Such limited concurrency will inhibit performance in multi-threaded applications." The proposed fix was to make "all iterator and element access operations safely concurrently executable." [0]. This involved limiting the circumstances in which iterators could be invalidated, which among other things meant that non-const operator[] could no longer invalidate iterators, which meant copy-on-write was no longer a viable implementation [1].
One nice thing to remember is for projects you control, you can dictate the ABI. There isn't a law enforcement bureau that will arrest you for passing parameters in whatever register suits you. You just have to imagine life differently than they did in the 70s when they dreamed up dynamic libraries for reasons that are largely no longer valid.
Does the C++ standard guarantee binary ABI? Isn't that something that compiler and library contributors handle separately? So I think the arguments you see online are more accurate—vendors are the ones maintaining it.
In other words, the ABI we rely on today isn't really part of the C+ standard—it's more like the Itnaium C++ ABI or the MSVC C++ ABI.
In the end, I think the ABI stays stable because of community conventions established by compiler vendors.
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.
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,...
One reason. The other is C is easy. Any language can implement it and have confidence it can work. (there are other options that are just as easy, but C won). C++ is much harder because of things like function overloading and exceptions - none of it is difficult, but there is a lot more to do and thus more opportunities for bugs.
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.
Afaik Herb Sutter largely stopped working on Cppfront a few months ago, and was never quite supposed to be a real thing, just a personal playground (or so Sutter claims at this point, anyway).
Afaik Carbon is at this point the only attempt at a successor language that's still going?
C++ famously has no ABI, it is the implementations of C++ that have ABIs. The reason the situation is so bad is that it's considered explicitly out of scope of the language (it targets an abstract machine).
Yes and no. The Linux kernel barely interacts with the C calling convention ABI (vDSO), and you can interact with the OS without going through a C library. I guess the layout of some structs are exposed through some binary protocols, but most of them are not C function calls.
C++ ABIs are why Win32 settled on things like COM to do cross-module passing of objects. Because nobody could agree on a standard, the COM standard enforced a very specific calling convention, and a very specific vtable layout for COM objects.
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.
I agree!
(Though you cannot call just any method asynchronously; the interface's IDL needs specific annotations and the underlying object needs to implement ICallFactory.)
Well said.
COM was one of the best realizations of "Component-based Software Engineering" and Brad Cox's "Software-IC" model. It was a binary standard and so components could be written in any language and yet be assured of perfect interoperability provided you followed all the rules/conventions. There was a bunch of boiler-plate for the framework itself but once you understood it, everything was smooth sailing in your language of choice.
One of the things i always advise people is not to focus only on the current way of doing things but to study older well-known libraries/frameworks/architectures/kernels/etc. to really understand "Software Engineering" from many perspectives. That is where insight comes from and real understanding happens.
For people interested in understanding COM, see the classic Essential COM by Don Box.
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
SOM predates COM, and is even cooler, as it supported class inheritance, metaclasses and Smalltalk as target language.
On OS/2, Smalltalk enjoyed a role similar to what would be .NET and VB on Windows.
Sure you can use delegation with aggregation, and type libraries (nowadays .NET metadata), but still isn't as ergonomic.
As always, read wikipedia and follow links as needed - https://en.wikipedia.org/wiki/Application_binary_interface
GNU libstdc++ "Dual ABI" issue - https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_a...
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.
The changes to std::string were made because "the [pre-C++11] definition of basic_string allows only very limited concurrent access to strings. Such limited concurrency will inhibit performance in multi-threaded applications." The proposed fix was to make "all iterator and element access operations safely concurrently executable." [0]. This involved limiting the circumstances in which iterators could be invalidated, which among other things meant that non-const operator[] could no longer invalidate iterators, which meant copy-on-write was no longer a viable implementation [1].
[0]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n26...
[1]: https://stackoverflow.com/q/12199710
One nice thing to remember is for projects you control, you can dictate the ABI. There isn't a law enforcement bureau that will arrest you for passing parameters in whatever register suits you. You just have to imagine life differently than they did in the 70s when they dreamed up dynamic libraries for reasons that are largely no longer valid.
Does the C++ standard guarantee binary ABI? Isn't that something that compiler and library contributors handle separately? So I think the arguments you see online are more accurate—vendors are the ones maintaining it.
In other words, the ABI we rely on today isn't really part of the C+ standard—it's more like the Itnaium C++ ABI or the MSVC C++ ABI.
In the end, I think the ABI stays stable because of community conventions established by compiler vendors.
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.
Thank you for the kind comment. I'll make a note of it and look into it.
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,...
Isn't this one reason why C is the de facto FFI? OSes happen to use it in their OS ABI and so everything is forced to speak it
One reason. The other is C is easy. Any language can implement it and have confidence it can work. (there are other options that are just as easy, but C won). C++ is much harder because of things like function overloading and exceptions - none of it is difficult, but there is a lot more to do and thus more opportunities for bugs.
OS written the C, which is what many overlook.
If you are on e.g. z/OS you would be using ILE, Integrated Language Environment, on ChromeOS JS/WASM, on Android either DEX or JNI,...
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.
Afaik Herb Sutter largely stopped working on Cppfront a few months ago, and was never quite supposed to be a real thing, just a personal playground (or so Sutter claims at this point, anyway).
Afaik Carbon is at this point the only attempt at a successor language that's still going?
Probably, but note that Google says they are they main customer anyway, and it is an experiment above anything else.
Anyone that can reach out to Rust, Go, Java, C#, whatever, should do that preferably.
There are also some efforts to tame existing C++ with profiles, and replacing UB with erroneous behaviour, and that's about it.
C++ famously has no ABI, it is the implementations of C++ that have ABIs. The reason the situation is so bad is that it's considered explicitly out of scope of the language (it targets an abstract machine).
Neither of the ISO C and ISO C++ standards define an ABI.
Right, C doesn't specify an ABI either but (many) OSes do and they provide their ABIs in C so C became the defacto ABI of a given platform.
Yes and no. The Linux kernel barely interacts with the C calling convention ABI (vDSO), and you can interact with the OS without going through a C library. I guess the layout of some structs are exposed through some binary protocols, but most of them are not C function calls.