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

[David Chisnall via cfe-dev]

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.

David