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

[James Gregurich via cfe-dev]

> On Mar 28, 2018, at 3:07 PM, John McCall <rjmccall at apple.com> wrote:
> 
> I think we actually take the parameter as a pr-value, which in some ways is better because it means that you can pass something either by copy or by move.  I don't know if we then try to move-assign the ivar from the parameter, but we certainly should.
> 
> I think the more interesting question is the type of the getter.  The getter really can't return a && or pr-value.  Do we recognize that a type is move-only and just make the getter return a `const &`?  Does that mean we forbid synthesizing such a property as atomic?


Let me focus on issues I encounter that make @property problematic or inconvenient. perhaps I'm misusing the tool.


consider the code below.  What I want to be able to do is get a reference to the map instance variable and modify it. but the code doesn't compile because the property wants to return a  copy. 


@interface AppDelegate ()
@property (nonatomic, assign) std::map<std::string, int> prop;
@end

@implementation AppDelegate
@synthesize prop;


- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
   auto& tmpP = self.prop;
   tmpP["test"] = 1;
}


@end


ok. so I try this... 


@property (nonatomic, assign) std::map<std::string, int>& prop;


but then I encounter the following that doesn't work...

self.prop = std::map<std::string, int>{{"test",1}, {"test2",2}};

and of course this doesn't compile...


@property (nonatomic, assign) std::map<std::string, int>&& prop;


The only answer I see here is to manually write the accessor functions and have an explicit instance variable declaration. I'd like to be able to use @property for convenience, but I see no way to make it work for what I naturally want.  is there an answer that isn't "don't use @property"?

-James





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