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Re: Which ObjC2.0 features are missing in the latest GCC?


From: Pirmin Braun
Subject: Re: Which ObjC2.0 features are missing in the latest GCC?
Date: Tue, 26 Nov 2019 11:09:33 +0100

I'd suggest a fork, i.e. "Gnustep2" with LLVM, Clang, libobjc2 

On Tue, 26 Nov 2019 04:55:43 -0500
Gregory Casamento <address@hidden> wrote:

> I'd really like some resolution on this topic.   There seem to be a lot of
> reasons for and against.
> 
> GC
> 
> On Mon, Nov 25, 2019 at 1:04 PM David Chisnall <address@hidden>
> wrote:
> 
> > On 25 Nov 2019, at 14:07, H. Nikolaus Schaller <address@hidden> wrote:
> > > I am not sure that this is the only way to implement it.
> > >
> > > First of all the callMethodOn returns some block which is a data
> > structure knowing that it should take the parameter x and do some function.
> > > Let's call it NSBlock. NSBlock can be an ordinary object like any other
> > so that it can follow the same memory management rules as used otherwise.
> >
> > That’s shifting the goalposts somewhat.  It is not news that objects and
> > closures are equivalent.  Smalltalk implemented blocks as BlockClosure
> > objects, Ian Piumarta’s Composite Object-Lambda Architecture, and C++
> > lambdas (which are just shorthand for C++ objects that implement
> > `operator()`).  You can always express anything that uses blocks with
> > objects.
> >
> > There are two issues:
> >
> > 1. If you want to be compatible with existing APIs that use blocks, you
> > need to be ABI compatible with blocks.
> > 2. The reason that most languages that have objects also have blocks is
> > that the shorthand syntax is very convenient.
> >
> > The following are roughly equivalent:
> >
> > ```
> > @interface Delegate : NSObject
> > - (void)invoke;
> > - (instancetype)initWithCapture: (id)someObject;
> > @end
> >
> > @implementation Delegate
> > {
> >         @private
> >         id obj;
> > }
> > - (instancetype)initWithCapture: (id)someObject
> > {
> >         if ((self = [super init]) == nil) return nil;
> >         obj = [someObject retain];
> >         return self;
> > }
> > - (void)invoke
> > {
> >         [obj doSomething];
> > }
> > - (void)dealloc
> > {
> >         [obj release];
> >         [super dealloc];
> > }
> > @end
> >
> > // At construction site:
> >
> > [[Delegate alloc] initWithCapture: x];
> >
> > // At use site:
> >
> > [delegate invoke];
> > ```
> >
> > And this, with blocks:
> >
> > ```
> > // At construction site:
> >
> > ^() { [x doSomething]; };
> >
> > // At use site:
> >
> > delegate();
> > ```
> >
> > At use, these are similar complexity for the programmer.  At the point of
> > construction, one is one line of code (two or three if you put lambda
> > bodies on their own lines), the other is 26.  As a programmer, I don’t want
> > to write 26 lines of code for a one-line callback.
> >
> > In C++98 you could probably template that and provide a generic class that
> > took a struct containing the captures and a C function, so you’d get a lot
> > less boilerplate.  Assuming you had fudged ARC like this (as above, this
> > code is typed into a mail client and probably doesn’t compile):
> >
> > ```
> > template<typename T>
> > struct ObjCObjectWrapper
> > {
> >         ObjCObjectWrapper(T x) : obj(objc_retain(x)) {}
> >         ObjCObjectWrapper(const ObjCObjectWrapper &other) :
> > obj(objc_retain(other.obj) {}
> >         ObjCObjectWrapper(ObjCObjectWrapper &&other) : obj(other.obj)
> >         {
> >                 other.obj = nil;
> >         }
> >         ObjCObjectWrapper()
> >         {
> >                 objc_release(obj);
> >         }
> >         operator=(T x)
> >         {
> >                 objc_storeStrong(&obj, x);
> >         }
> >         T operator()
> >         {
> >                 return obj;
> >         }
> >         private:
> >         T obj;
> >
> > };
> > ```
> >
> > You could then define a generic capture structure and invoke method like
> > this:
> >
> > ```
> > template<typename Capture, typename Ret, typename... Args>
> > struct BlockImpl
> > {
> >         using invoke_t = Ret(*)(Capture &, Args...);
> >         void operator()(Args... args)
> >         {
> >                 inv(capture, std::forward<Args>(args)…);
> >         }
> >         Block(Capture &&c, invoke_t fn) : capture(c), inv(fn) {}
> >         private:
> >         Capture capture;
> >         invoke_t inv;
> > };
> > ```
> >
> > This is then generic and you could use it as follows:
> >
> > ```
> > struct CaptureOneObject
> > {
> >         ObjCObjectWrapper<id> o;
> > };
> > void invoke(CaptureOneObject &c)
> > {
> >         [(id)c.o doSomething];
> > }
> > // At construction site:
> > std::function<void(void)> block(BlockImpl<CaptureOneObject, void>({x},
> > invoke));
> > // At use site:
> > block();
> > ```
> >
> > I *think* you could get the same ABI as blocks if you worked on the
> > generic templated boilerplate a bit.
> >
> > Of course, if you were using C++ then you could also write it using
> > lambdas as:
> >
> > ```
> > // At construction site
> > ObjCObjectWrapper<id> capture(x);
> > auto block = [=capture]() { [(id)capture.o doSomething]; };
> > // At use site:
> > block();
> > ```
> >
> > And with this you don’t need the invoke function or the capture class.
> > Again, much less boiler plate for users, though we don’t have ABI
> > compatibility with blocks.
> >
> > If you were using ARC and C++, then this reduces even further to:
> >
> > ```
> > auto block = [=]() { [x doSomething]; };
> > ```
> >
> > And now we’re back with different syntax for the same thing, though with a
> > different ABI (I think Clang has support for implicitly converting C++
> > lambdas to blocks, but it’s been a few years since I tried)
> >
> > David
> >
> >
> >
> 
> -- 
> Gregory Casamento
> GNUstep Lead Developer / OLC, Principal Consultant
> http://www.gnustep.org - http://heronsperch.blogspot.com
> http://ind.ie/phoenix/


-- 
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