[cfe-dev] objc++ enhancements for new c++ features

[John McCall via cfe-dev]

> On Mar 29, 2018, at 2:31 PM, David Chisnall <David.Chisnall at cl.cam.ac.uk> wrote:
> On 29 Mar 2018, at 19:22, John McCall <rjmccall at apple.com> wrote:
>> 
>>> On a related note, I’ve been considering whether it’s possible to provide helpers to improve NSInvocation’s handling of Objective-C++ messages that take types with either nontrivial copy constructors or no copy constructor.
>>> 
>>> My motivation for this was writing some code using higher-order messaging to perform things in another thread, which worked fine until I passed it a std::shared_ptr, at which point NSInvocation did a simple memcpy of the argument frame and I was left with a dangling pointer.  I eventually rewrote this code to use lambdas, but it would be nice if an NSInvocation constructed from a message containing a std::shared_ptr argument actually did the reference count manipulations correctly when copying the argument from the stack / registers into the NSInvocation object.  Most methods have trivially copyable arguments / return values (except for Objective-C objects, which NSInvocation can already handle automatically), so the metadata would only have to be for the (hopefully, relatively small) subset of C++ types that are not trivially copyable and are passed as Objective-C method arguments.  Do you have any thoughts?
>> 
>> Honestly?  I think NSInvocation ought to be deprecated in favor of just using blocks or, if blocks are unacceptable, static code-generation techniques.  NSInvocation works well enough for a highly reduced set of function prototypes, but the idea that it's going to transparently support every possible thing that the compiler knows how to do in a call seems pretty fanciful to me.
> 
> Unfortunately, using blocks means that you can’t use any of the higher-order messaging patterns, which makes the code a lot more verbose and a lot less composable.  If C++ compile-time reflection lands then I suppose most use cases could be handled with that, as long as we provide ObjC++ extensions to allow the reflection to work in the same way.

Well, but does that mean that blocks aren't the right answer, or does it mean that you should be generating your blocks with code-generation techniques like macros or templates?

John.