Swift 5 represents a stable ABI. That means from this point forward all binaries built with Swift 5 or later will run against a newer Swift stdlib.
The ABI is entirely separate from source compatibility. Swift is already source compatible and the bar for a source-breaking change is extremely high.
I'm pretty sure C++ doesn't prescribe any particular ABI for implementations to adhere to. There was even a paper in 2014 about defining a portable ABI, and I don't believe it was adopted [0].
In particular, MSVC uses a different ABI than the *nixes, and MSVC's ABI isn't guaranteed to be stable across major compiler releases. In addition, libstdc++ 5.1 had a significant break for C++11 std::string and std::list [1].
[0]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4028.pdf
[1]: https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.htmlThe number 5 _also_ implies significant breaking changes. Otherwise, why wouldn't it be Swift 4.3?
Removing Swift 3 compatibility mode (which was part of Swift 4) is itself a major source-compatibility breaking change, even if nothing else.
But that's an unfair comparison since its "binary" format is non-native.
C++ implementations generally use a calling convention that is stable ABI on each platform, but different platforms may have a different ABI. In the past the MSVC ABI was different from most posix platforms, I’m not sure if that carries through to win64.
The easiest way to explain why C++ the language doesn’t prescribe the calling convention ABI is that you may want to compile code on different cpu architectures. There’s no way you could (sensibly) have the same calling convention :)
Huge pain for closed-source libraries.
ABI stability at a language level is for code (replace syntax with your language of choice):
class Foo { virtual void bar();
int _i;
float _f;
bool b;
int bitfield : 5;Should be stable across multiple compilers and versions. This is what swift currently does not have.
This is vs. source defined ABI, e.g.
class Foo { virtual void bar(); virtual void wibble(); int i; float f; }
vs.
class Foo { virtual void wibble(); virtual void bar(); float f; int i; }
In this example Foo is API stable, and the language ABI means either definition is ABI stable, but as a publicly exposed API it is not ABI stable. C++ defines vtable layout in terms of order of definition (so changing the order of virtual methods is not stable). And you have the source compatible reordering of i and f, which is just as ABI breaking in C++ as it is in a C struct.
This is all about ABI - can binaries compiled with one version of the compiler interoperate with binaries compiled with a different version.
API stability is easy - C++ has an extremely stable API. Swift is pretty stable apparently.
SomeLibrary.h:
class Foo {
virtual void bar();
virtual ~Foo();
The library is compiled into SomeLibrary.dll,dylib,so or what have you.
Your program uses this:
void my_cool_function(Foo* foo) {
foo->bar();
Later on the library makes a change, and adds a new virtual method wibble(). The API is stable -- they haven't removed Foo::bar, and they haven't changed its parameters or return type. But this API change can break the library ABI if it declared the new virtual method in the wrong part of the class, because vtable ordering is defined as being order of declaration.
So there are two aspects of ABI compatibility. First, does a given API result in the same ABI irrespective of what compiler version it uses. The second is, given that your language is ABI stable, are your changes to the API also ABI stable. C++ is notoriously bad at this, because of foot guns like vtable ordering, changes to binary layout of superclasses, etc.
Swift is at least in existing versions even less stable, where you're not necessarily guaranteed that a single API will get the same ABI depending on what things it is compiled with, and certainly not through even minor compiler version changes - let alone actual language version changes.
Is this really true? Looking through 4.0 release notes I see some incompatible changes like:
SE-0164 Remove final support in protocol extensions
SE-0160 Limiting @objc inference
It's not the Great Renaming, and it might not be fair to the Swift project itself, but unless I can compile a project using the 10.13 and 10.14 SDKs without source changes (which you can in Objective-C), the impression is that Swift doesn't have source compatibility.
Yes, you can continue compiling v3 code with a v4 compiler with a flag, but if you compile your v3 code without a flag, your code may fail to compile. So while there may not be a compiler breakage, there is a language breakage between v3 and v4, and apparently there will be another one between v4 and v5.
> When the Swift 4.2 compiler is working with Swift 3 code, it identifies its language version as 3.4. As a result, you can use conditional compilation blocks like #if swift(>=3.4) to write code that’s compatible with multiple versions of the Swift compiler.
Swift 2 to 3 was a little bit of work but the new succinct syntax was worth it.
It doesn’t sound like there will be many breaking changes in 5.0:
“As with Swift 4.2, the vast majority of sources that built with the Swift 4.2 compiler should compile with the Swift 5.0 compiler”
It wasn't even major. I updated several projects from Swift 3 to 4 without issue. The largest project was close to 200k loc and it only took me 20 minutes to get everything up to date.
Just make sure your dependencies aren't in Swift because then you're relying on 3rd parties to update their code. I'll be avoiding all Swift libraries except for PromiseKit (it's so nice!).
Compare this to Go (no major revisions after 6 years, probably a backwards-compatible major revision in 2-3 years) and Rust (no major revisions after 3 years, but with a mechanism called "editions"). To say nothing about Java or C#.